Carbon Fibers Having Improved Strength and Modulus and an Associated Method and Apparatus for Preparing Same

ABSTRACT

The invention is directed to carbon fibers having high tensile strength and modulus of elasticity. The invention also provides a method and apparatus for making the carbon fibers. The method comprises advancing a precursor fiber through an oxidation oven wherein the fiber is subjected to controlled stretching in an oxidizing atmosphere in which tension loads are distributed amongst a plurality of passes through the oxidation oven, which permits higher cumulative stretches to be achieved. The method also includes subjecting the fiber to controlled stretching in two or more of the passes that is sufficient to cause the fiber to undergo one or more transitions in each of the two or more passes. The invention is also directed to an oxidation oven having a plurality of cooperating drive rolls in series that can be driven independently of each other so that the amount of stretch applied to the oven in each of the plurality of passes can be independently controlled.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 11/562,867, filed Nov. 22, 2006, which is hereby incorporated hereinin its entirety by reference.

FIELD OF THE INVENTION

The present invention relates generally to carbon fibers and moreparticularly to carbon fibers having improved strength and modulus, anda method and apparatus for making the carbon fibers.

BACKGROUND OF THE INVENTION

Carbon fibers have been used in a wide variety of structuralapplications and industries because of their desirable properties. Forexample, carbon fibers can be formed into a structural component thatcombines high strength and high stiffness, while having a weight that issignificantly lighter than a metal component of equivalent properties.Carbon fibers can be manufactured by converting a precursor fiber, suchas a spun polyacrylonitrile (PAN) fiber, in a multi-step process inwhich the precursor fiber is heated, oxidized, and carbonized to producea fiber that is 90% or greater carbon. The resulting carbon fibers canbe molded into high strength composite materials for structuralapplications, used in their pure form for electrical and frictionapplications, or can be further processed for use in adsorbent, filter,or other applications. In particular, composite materials have beendeveloped in which carbon fibers serve as a reinforcing material in aresin, ceramic, or metal matrix.

Increasingly, carbon fibers are being used as structural components inaerospace applications. In order to meet the rigorous demands of theaerospace industry, it is necessary to continually develop new carbonfibers having both high tensile strength and high modulus of elasticity.In particular, there is a desire to develop carbon fibers having atensile strength of 1,000 ksi or greater and a modulus of elasticity of50 Msi or greater. Carbon fibers having individually higher tensilestrength and modulus can be used in fewer quantities than lower strengthcarbon fibers and still achieve the same total strength for a givencarbon fiber composite component. As a result, the composite componentweighs less. A decrease in component weight is important to theaerospace industry and increases the fuel efficiency of aircraftincorporating such a component.

Several methods of increasing tensile strength and modulus have beenexplored in the prior art, and have generally had mixed results. Forexample, it is generally known that modulus can be increased byincreasing carbonization temperatures. However, increases incarbonization temperatures result in a decrease in tensile strength. Asa result, this method has generally not provided an effective means forpreparing carbon fibers having improved tensile strength and modulus ofelasticity.

Other methods have focused on stretching the precursor fibers before orduring the process of converting the precursor fiber to a carbon fiber.It has previously been recognized in the prior art that the modulus ofcarbon fibers can be improved by stretching the fibers in apost-spinning step, oxidizing step, carbonizing step, or a combinationthereof. However, conventional wisdom believed that the amount ofstretching in the oxidizing step was limited by tension levels in thefibers that developed in response to the onset of chemical reactions,such as thermally induced cyclization and/or oxidative crosslinking ofthe PAN precursor fibers. The accumulation of tension caused the fibersto break at relatively low stretches under standard oxidationconditions, e.g., above 180° C. As a result, prior attempts to stretchPAN fibers during oxidation have generally been limited to a maximumamount of stretch or to a single continuous stretch.

Several studies and prior art references have further indicated thatimprovements beyond this initial or maximum stretch provide little ifany gain in properties, and in fact may actually lead to breakage ordamage in the fibers. For example, U.S. Pat. No. 4,609,540 describes amethod of determining the optimum stretch to be applied to a precursorfiber in an oxidizing atmosphere. According to the '540 patent, theoptimum amount of stretch corresponds to an inflection point that isdeterminable from a plot of % elongation versus tension, and that thisoptimum elongation also roughly corresponds to the maximum degree ofcrystalline orientation within the fibers. Beyond this inflection point,the '540 patent teaches that any gains from further stretching areminimal and may result in the development of fluff and possiblybreakage.

Thus, there exists a need for carbon fibers having both high tensilestrength and high modulus of elasticity, and for a method and apparatusthat can be used to prepare such carbon fibers.

BRIEF SUMMARY OF THE INVENTION

The present invention provides carbon fibers having improved strengthand modulus and a method and apparatus that can be used to prepare thecarbon fibers. In one embodiment, the method comprises advancing aprecursor fiber through an oxidation oven wherein the fiber is subjectedto controlled stretching in an oxidizing atmosphere in which tensionloads are distributed across a plurality of passes through the oxidationoven. As a result, the overall cumulative stretch of the fiber can beincreased by selecting stretch conditions that permit distribution ofthe tension loads over multiple passes. Distributing tension loadsamongst a plurality of passes permits the fiber to be stretched to anextent greater than previously expected. This controlled stretching ofthe fibers during oxidation can help provide, for example, improvementsin orientation, uniformity in oxidation, and reduction in the growth offlaw-inducing crystallites, which in turn can provide improvements inthe modulus of elasticity and tensile strength of the resulting carbonfibers.

In one embodiment, the method comprises passing a carbon fiber precursorpolymer through an oxidation oven in which the fiber is subjected aplurality of controlled stretches so that the fiber is subjected to a %stretch in at least one pass that is between 5 and 30% and a % stretchin subsequent passes that is between 5 and 20%, and 2 to 15% stretch. Inone particular embodiment, the fiber is subjected to a % stretch in afirst pass that is between 5 and 30%, a % stretch in a second pass thatis between 5 and 20%, and a % stretch in a third and fourth pass that isbetween 2 and 15%. In a further embodiment, the method comprisesstretching a carbon fiber precursor fiber to a plurality of controlledstretches in the oxidation oven wherein: a) in a first pass the fiber issubjected to a % stretch that is between 10 and 40%; b) in a second passthe fiber is subjected to a % stretch that is between about 2 and 20%;c) in a third pass the % stretch is between about 2 and 16%; and d) in afourth pass the fiber is subjected to a % stretch that is between about2 and 12%. After the oxidation step is completed, the thus oxidizedfiber can then be passed through a furnace at a temperature betweenabout 400 and 800° C. followed by carbonizing the fiber by passing thefiber through a furnace having a temperature that is between 1300 and1500° C.

In a further aspect of the invention, it has also been discovered thatimprovements in tensile strength and modulus of elasticity can beachieved by controlled stretching of the fiber in an oxidizingatmosphere in which the fiber is subjected to an amount of stretch thatcauses the fibers to undergo one or more transitions. In an exemplaryembodiment, carbon fibers are prepared by advancing a precursor fiberthrough an oxidation oven in a plurality of passes in which theprecursor fiber is subjected to a controlled amount of stretching in twoor more of the passes so that precursor fiber undergoes at least twotransitions in each of the two or more passes. The transitions comprisea region of inflection that is determinable from a plot of tensionversus % stretch for a given pass. In some embodiments, the precursorfibers may be subjected to a controlled amount of stretching in whichthe fiber is advanced through a plurality of passes through an oxidationoven, for example from 2 to 20 passes.

In another aspect, the invention is directed to an oxidation oven thatis capable of subjecting a precursor fiber to a plurality of controlledstretching passes in an oxidizing atmosphere. In one embodiment, theoxidation oven includes a plurality of drive rolls and a plurality ofidler rolls, wherein a drive roll and idler roll cooperate to define afiber pass through the oxidation oven. In one embodiment, the driverolls can be driven independently of each other so that the speed, andalternatively the tension, on at least two or more of the passes throughthe oxidation oven can be independently controlled. In some embodiments,the idler rolls include a tension measuring device, such as load cell,that permits continuous monitoring of fiber tension as the fiber isbeing advanced through the oxidation oven.

After the oxidizing step, the remainder of the process for convertingthe fibers into carbon fibers can be carried out utilizing conventionalmethods. The fibers can be converted by advancing the oxidized fibersthrough a low temperature and a high temperature furnace. In oneembodiment, controlled stretching of the fibers during oxidizationpermits further stretching of the fibers as they are advanced throughthe low temperature furnace by an amount, for example, between 5 and 40percent.

Carbon fibers prepared in accordance with the invention can have amodulus of elasticity that approaches and exceeds 50 Msi and a tensilestrength that approaches and exceeds 1,000 ksi. In one embodiment, theinvention provides a carbon fiber having a tensile strength of at least950 ksi and modulus of elasticity of at least 45 Msi, and wherein theatomic force microscopy (AFM) surface image of the carbon fiber ischaracterized by the presence of a plurality of striations of low phaseangle domains and a plurality of striations of high phase angle domainsthat extend across the surface of the carbon fiber. In addition, carbonfibers prepared in accordance with the invention may have a ArithmeticAverage Roughness (Ra) value that is greater than about 2.0, and inparticular greater than 2.5, and more particularly greater than about3.0, and a Root Mean Square Roughness (Rq) value that is greater thanabout 2.0, and in particular greater than 3.0, and more particularlygreater than about 4.0. Further, carbon fibers prepared in accordancewith the invention may have an average phase angle depth of 5 nanometersor greater. In some embodiments, the carbon fibers may have an averagephase angle depth of 8 nanometers or greater and in particular, 10nanometers or greater.

In one embodiment, carbon fibers prepared in accordance with theinvention have L_(a) values that are about 4 nm or greater, and inparticular greater than about 4.5 nm, and in some embodiments greaterthan 5.0 nm. Carbon fibers prepared in accordance with the invention canalso be characterized by the combination of high modulus and resistivityvalues. For example, in one embodiment, the carbon fibers can have amodulus of elasticity of at least 50 Msi, a resistivity of at least 13μΩ·m or greater.

Thus, the invention provides carbon fibers having improved tensilestrength and modulus of elasticity, and a method and apparatus formaking such carbon fibers.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a graphical illustration of a reaction process in which a PANprecursor fiber undergoes cyclization and oxidation to form a pyridonestructure;

FIG. 2 is a graphical illustration of an exemplary oxidation oven thatmay be used in accordance with the invention;

FIG. 3 is a schematic illustration of a system that can be used toconvert a precursor fiber into a carbon fiber;

FIG. 4 is a graphical plot of tension vs. % stretch that depicts a fiberundergoing multiple transitions as it is being stretched;

FIG. 5 is a graphical plot of the first derivative of tension vs. %stretch that highlights the transition points;

FIG. 6 is a graphical plot of the second derivative of tension vs. %stretch that highlights the transition points;

FIGS. 7A and 7B are Atomic Force Microscopy Image (AFM) of carbon fibersthat are prepared in accordance with a prior art method of stretchingcarbon fibers in an oxidation oven; and

FIGS. 7C through 7F are Atomic Force Microscopy Images (AFM) of carbonfibers that are prepared in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the invention are shown. Indeed, these inventions may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

In one aspect, the present invention is directed to carbon fibers havingimproved tensile strength and modulus of elasticity. In another aspect,the invention is directed to an apparatus and method of making thecarbon fibers. Carbon fibers prepared in accordance with the method ofthe invention can have a tensile strength approaching and exceeding 1000ksi and a modulus of elasticity approaching and exceeding 50 Msi.

As discussed in greater detail below, carbon fibers in accordance withthe invention can be prepared by subjecting a precursor fiber, such as afiber comprising polyacrylonitrile (PAN), to a plurality of passesthrough an oxidizing atmosphere in which the fiber is controllablystretched in two or more of the passes through the oxidizing atmosphere.Upon completion of the oxidizing step, the fibers can be advancedthrough one or more additional furnaces, such as a low temperaturefurnace and a high temperature furnace, to complete conversion of theprecursor fibers into carbon fibers. In the context of the invention theterm “fiber” includes a single filament or a plurality of filaments thatare bundled together, also referred to as a tow. A tow or bundle mayinclude from about 1,000 to 100,000 individual filaments.

In the context of the invention, the term “precursor fiber” refers to afiber comprising a polymeric material that can, upon the application ofsufficient heat, be converted into a carbon fiber having a carboncontent that is about 90% or greater, and in particular about 95% orgreater, by weight. The precursor fiber can comprise both homopolymersand copolymers of acrylonitrile (AN), and may include copolymers such asmethyl acrylate (MA), methacrylic acid (MAA), sodium methallylsulfonate,itaconic acid (IA), vinyl bromide (VB), isobutyl methacrylate (IBMA),and combinations thereof. In one embodiment, the precursor fibercomprises a polyacrylonitrile (PAN) polymer formed primarily fromacrylonitrile monomers.

The precursor fibers can be prepared by melt spinning of by solvatingthe precursor polymers in organic and/or inorganic solvents such asdimethylsulfoxide, dimethyl formamide, zinc chloride or sodiumthiocyanate solutions to form a spinning solution. In a particularembodiment, the spinning solution is formed from water, acrylonitrilepolymer and sodium thiocyanate at exemplary respective weight ratios ofabout 60:10:30. This solution can then be concentrated throughevaporation and filtered to provide the spinning solution. In oneembodiment, the spinning solution comprises about 15% by weight of theacrylonitrile polymer. The spinning solution is passed throughspinnerets using conventional spinning processes, such as dry, dry/wetor wet spinning, to form the polyacrylonitrile precursor. In aparticular embodiment, PAN precursor fibers are made using a dry/wetspinning wherein a multitude of filaments are formed from the spinningsolution and pass from the spinneret through an air gap or other gapbetween the spinneret and a coagulant, such as aqueous sodiumthiocyanate. After exiting from the coagulant bath, the spun filamentsare washed. In some embodiments, the spun filaments can be stretched upto several times their original length in hot water and steam. (Seee.g., U.S. Pat. No. 4,452,860, which is incorporated herein byreference.) In addition, the polyacrylonitrile precursor fiber can betreated with sizing agents, such as silane compounds, to improve itshandling during manufacture of the carbon fiber. Exemplary methods ofpreparing PAN precursor fibers are discussed in greater detail in U.S.Pat. No. 5,066,433, the contents of which are incorporated herein byreference.

The precursor fibers can comprise polyacrylonitrile based fibers thatare made from between about 85 and 99% by weight acrylonitrile andbetween about 15 and 1% of other monomers such as methacrylic acid,acrylic acid, methyl acrylate, and methyl methacrylate, and combinationsthereof. The polyacrylonitrile precursor fibers are in the form ofbundles that each comprise between about 3000 and 50,000 filaments perbundle, and in particular between about 3000 and 24,000 filaments perbundle. The filaments may have a mean average denier between about 0.50and 1.50, and in particular between about 0.60 and 0.85, preferably with95% or more of the filaments in each bundle having differences in denierwithin ±0.05 dpf. In one embodiment, the polyacrylonitrile startingmaterial has a smooth surface, a round cross section, and an intrinsicviscosity of between about 1.5-2.5 deciliters per gram. Filamentdiameters prior to conversion may range from about 7.5 to 13.5 μm, andmore generally from about 8.5 to 10.5 μm.

During oxidation, which is also referred to as oxidative stabilization,the PAN precursor fibers are heated in an oxidizing atmosphere at atemperature between about 150° to 600° C. to cause the cyclization andoxidation of the PAN precursor molecules. In this regard, FIG. 1illustrates in a step-wise manner the process of cyclizing and oxidizingthe PAN precursor fibers. In step (A), the nitrile groups of the PANbecome aligned. In steps (B) and (C), the nitrile groups polymerize toform a polynaphthyridine ring “ladder” structure, which undergoestautomerization in step (D) to form a polycyclic dihydropyridine. Instep (E), the polycyclic dihydropyridine undergoesoxidation/dehydrogenation to form stabilized pyridone structures.

During oxidation, the extent to which the reactions illustrated in FIG.1 occur for a given precursor are generally a function of temperatureand filament diameter. This is believed to be due in part to theinfluence of oxygen diffusion into the filaments. At relatively lowtemperatures (e.g., about 240° C. or less) and/or relatively smallfilament diameters (e.g., about 10 microns or less) the rate of oxygendiffusion into the filament cores is promoted relative to the rate ofoxygen reaction with the filament surfaces. At higher temperaturesand/or larger filament diameters, oxygen tends to react faster than itis able to diffuse, and a skin layer of oxidized fiber is formed arounda core where only thermally induced reactions take place. The oxidizedsurface layer is thought to act as a diffusion barrier for oxygentransport into the filament cores. Its presence is undesirable becauseit leads to skin-core differences and both structural and chemicalinhomogeneities in the resulting fibers. For example, skin-coredifferences and structural inhomogeneities in the oxidized fibers canresult in the modulus of the outer layer being higher than that of theinner layer. This modulus distribution is caused by the difference inthe progression of cyclization/oxidation between the inner and outerlayers of the precursor fiber. The difference in the progression ofcyclization/oxidation is considered to result from a reduction in oxygenpermeation into the inner portions of the fiber due in part to theselective oxidation of the outer portions of the precursor fiber, whichleads to the formation of a barrier to oxygen diffusion into the fiber.

As noted above, carbon fibers of the invention can be prepared bypassing a precursor fiber through an oxidation oven in a plurality ofpasses in which the precursor fiber is subjected to controlledstretching in two or more of the passes through the oven. In oneembodiment, the controlled stretching includes independently controllingthe amount of tension that is applied to the fiber in two or more of thepasses through the oven. Independently controlling the stretching incontrolled passes through the oxidation oven provides severaladvantages. For example, in one embodiment, controlled stretchingpermits tension loads to be distributed among the plurality of passesthrough the oxidation oven, which results in reducing the maximum strainrate experienced by the fiber during processing. As a result, theprecursor fibers are able to be stretched to limits not previouslytaught in the prior art. This controlled stretching of the fibers duringoxidation can help provide, for example, improvements in orientation,uniformity in oxidation, reduction in the growth of flaw-inducingcrystallites, radial homogeneity, and the means to minimize or eliminatestructural flaws. These benefits can in turn provide improvements in themodulus of elasticity and tensile strength of the resulting carbonfibers.

With reference to FIG. 2, an exemplary oxidation oven that may be usedto controllably stretch a precursor fiber is illustrated and broadlydesignated as reference number 20. The oxidation oven includes aninterior 22 having an oxidizing atmosphere, such as air, that ismaintained at an elevated temperature between that is generally betweenabout 150 to 600° C., particularly between about 175 and 400° C., andmore particularly between about 175 and 300° C. In one embodiment, theprecursor fiber 24 is advanced through the interior of the oven in aplurality of passes in which the tension applied to the carbon fiber ineach of these passes can be independently controlled. In the context ofthe invention, the term “elevated temperature” means a temperature thatis high enough to cause oxidation of the PAN precursor fibers, but isnot so high as to cause undesirable effects in the fibers, such as thecreation of structural defects, burning, melting, or breaking of thefibers, and the like.

The oxidation oven 20 includes a plurality of idler rolls, collectivelyreferred to as reference number 28, and a plurality of drive rolls,collectively referred to as reference number 30. The precursor fiber 24is provided from a source, such as a creel (not shown), and is pulledforward by driven feed roll 26. Each idler roll 28 cooperates with oneor more corresponding drive rolls 30 to define a fiber pass through theoxidation oven. For the purposes of this invention, a “pass” is definedas the path traveled by the fiber from an upstream drive roll to adownstream drive roll, with at least some portion of the fiber travelingthrough the oxidation oven. A pass thus defined may include deflectionpoints in the form of idler rolls, bars, or other such devices. In theillustrated embodiment, a fiber pass refers to the path of the fiber asit travels between a drive roll and a corresponding drive roll. Forexample, the fiber path between roll 26 and drive roll 30 a defines asingle fiber pass through the oxidation oven.

In some embodiments, the oxidation oven may include pairs of cooperatingdrive rolls that define a fiber pass through the oxidation oven. In thisembodiment, a single fiber pass refers to the pathway of the fiber as ittravels through the oven between a first drive roll and a correspondingsecond drive roll.

In some embodiments, the precursor fiber 24 may exit the oxidation ovenbetween successive passes. In this regard, FIG. 2 illustrates anembodiment wherein the idler rolls 28 and the drive rolls 30 aredisposed on the exterior of the oxidation oven. Permitting the fiber toexit the oxidation oven between successive passes can help to dissipatesome of the exothermic heat released while stabilizing the PAN chains,and hence the fibers as they are being controllably stretched. Exteriorrolls may also help reduce the tendency of the fibers to stick onto hotsurfaces.

In other embodiments, the drive rolls, idler rolls or both may bedisposed in the interior of the oxidation oven. Also, it is notnecessary that idler rolls 28 be provided between successive drive rolls30, nor is it necessary that the passes be opposite one another. Forexample, assuming sufficient dwell time in the oven 20, the precursorfiber 24 may be driven in a straight line through the oven 20 by asuccession of drive rolls 30 nipped with idler rolls.

The drive rolls 30 can each be driven at speeds independent of the otherdrive rolls so that the amount of stretch or tension applied betweenpasses can be independently controlled. For example drive roll 30 a maybe driven at a speed V₁ that may be different or the same as the speedV₂ at which drive roll 30 b is driven. As a result, the tension appliedto the fiber 24 in the pass between rolls 28 a and 30 a, as well as the% stretch, can be different from the tension and % stretch applied tothe fiber 24 in the pass between rolls 30 a and 28 b. Independentlycontrolling the speed on each roll 30 permits the % stretch on each passto be independently controlled. As a result, the successive drive rollscan be used to distribute tensions or strain rates across a plurality offiber passes through the oxidation oven. In one embodiment, the fiber isexposed to a strain rate that is no greater than about 10% per minuteper pass.

In one embodiment, the drive rolls 30 are each separately in mechanicalcommunication with a motor for driving the rolls. Typically, the driverolls are each gear driven separately by an independent motor to provideimproved control over the speed at which the rolls are driven, and as aresult can provide improved control over the amount of tension that isapplied to the fiber. Although chain drives can be used in someembodiments, this is generally less desirable because of variations inspeed that may occur between the drive rolls.

The amount of drive rolls and idler rolls can be selected based on thedesired properties of the resulting carbon fibers. In one embodiment,the oxidation oven may include from 2 to 20 pairs of cooperating idlerand drive rolls. In other embodiments, the oxidation oven may includefrom 2 to 12 pairs of cooperating idler and drive rolls. In someembodiments, assemblies having more than one roll per inlet port, orconfigurations with rolls of different dimensions may be used toincrease the contact angle between the fiber and the rolls and thus helpreduce or eliminate slippage of the fiber during stretching. Forexample, a pair of rolls in close proximity to each other can define anS-shape in the fiber path, which can eliminate slippage of the fiber.

In some embodiments, the idler rolls 28 may include a tension measuringdevice, such as load cell, which permits the tension on each pass to becontinually monitored. The measured tension may then be used toseparately control the tension applied to the precursor fiber in a givenpass by adjusting the speed of the drive rolls with respect to eachother.

Stretch for a given pass is calculated from the difference betweenoutlet speed (V₂) and inlet speed (V₁) of successive drive rolls usingEquation 1:

% Stretch=100(V ₂ /V ₁−1)  (1)

For example, a 50% stretch can be attained if the ratio of relativespeeds (outlet/inlet)=1.50. Stretch can be adjusted by increasing V₂relative to V₁, decreasing V₁ relative to V₂, or varying both speedssimultaneously, until the ratio V₂/V₁=1.50. Note that a 50% stretchcorresponds to a stretch ratio of 1.50. In the context of the presentinvention, a 50% stretch is referred to as “1.5X”, and “2X” stretchimplies 100% stretch in comparison to the original length (1X) of thefiber. A “3X” stretch represents 200% stretch over the original length(i.e., three times as long as the original).

Upon exiting the oxidation oven, the fiber 24 can be advanced downstreamto one or more additional oxidation ovens, an intermediate furnace, orthe carbonization furnace. In this regard, FIG. 3 is a schematicillustration of a system and process that may be used in the conversionof the precursor fibers into carbon fibers. As shown, the precursorfiber 24 is provided via a supply roll 40. Alternatively, the precursorfiber can be provided from a plurality of precursor bundles that arecombined into a single bundle with a creel. The precursor fiber is thenpassed through one or more oxidation ovens 20 where it is subjected tocontrolled stretching.

In some embodiments, the system may include a plurality of oxidationovens where successive ovens are maintained at a temperature that isgenerally at least as great as that of a preceding oxidation oven. Whenthe system includes a plurality of oxidation ovens having an ascendingtemperature gradation, the temperature of successive ovens is typicallybetween about 1° to 50° C. higher than the temperature of a precedingoven, and more typically from 5° to 20° C. higher. In some embodiments,a temperature gradient may be set up in a single oxidation oven by meansof different heating zones within the oven. In other embodiments, theoxidation process can be carried out in environments wherein the oxygenconcentration is richer or leaner than that of atmospheric air. In stillother embodiments, oxidation processing steps may be preceded orinterjected by non-oxidizing gas treatments, or may be enhanced by theaddition of various stabilization promoters, flow pattern arrangements,and other methods known in the art.

After passing through the oxidation oven or ovens, the stretched,stabilized fiber is then passed through a low temperature furnace 42 orfurnaces, also referred to as the tar removal furnace, followed bypassage through a higher temperature furnace 44 or furnaces, alsoreferred to as a carbonization furnace. The low and high temperaturefurnaces contain an inert gas such as nitrogen. The temperature of thestabilized fiber in the low temperature furnace or furnaces rangesbetween about 300° C. and 900° C., and more typically between 400° C.and 800° C.

The low temperature furnace is purged of volatile products issuing fromthe passing stabilized fiber undergoing carbonization. After leaving thelow temperature furnace or furnaces, the fiber is then exposed to stillhigher temperatures e.g. between about 1200° C. and 2000° C., and inparticular between 1250° C. and 1600° C. in the high temperature furnaceor furnaces. In a preferred embodiment, the high temperature furnace isbetween about 1300 to 1500° C.

During travel through the low and high temperature furnaces the fibercan be subjected to further stretching so that its length is betweenabout 1 and 40%, such as between 1 and 30%, and in particular betweenabout 1 and 24%, longer upon its exit as compared to what it was uponentry. After completion of carbonization, the carbonized fiber may thenbe subjected to one or more further treatments including graphitization,surface treatments and/or sizing. Graphitization refers to heattreatments in one or more inert gas furnaces at temperatures exceeding2000° C. Surface treatments include anodic oxidation in which the fiberis passed through one or more electrochemical baths. Surface treatmentsmay aid in improving fiber adhesion to matrix resins and hence compositeproperties, as reflected by tests such as fiber-matrix interlaminar orshort beam shear strength assessment. Sizing typically involves passingthe fibers through a bath containing a water-dispersible material thatforms a surface coating or film to protect the fiber from damage duringits use. In composite applications, the water-dispersible material isgenerally compatible with matrix resin targeted for compositemanufacture.

As previously discussed, it has been commonly accepted that the amountof % stretch that can be applied to the fiber during oxidation islimited by the accumulation of tension loads in the fibers. However, theApplicant has discovered that the overall cumulative % stretch of thefiber can be increased by distributing the tension loads or stretches ina plurality of passes through the oxidation oven. As a result, theoverall cumulative stretch of the fiber can be increased by selectingstretch conditions that permit distribution of the tension loads overmultiple passes. In other words, by reducing the strain rate in a givenpass, the overall cumulative stretch can be increased without acorresponding increase in the maximum % stretch in the pass. Thispermits a higher degree of stretching to occur over the course ofmultiple passes, which can further help to improve the tensile strengthand modulus of the resulting carbon fibers. In the context of theinvention the term “cumulative stretch” refers to the overall % stretchof the fiber in comparison to the fiber prior to entering the oxidationoven. Cumulative stretch can be calculated from either the product ofthe stretches at each individual step or from the ratio of initial andfinal speeds within the section of interest.

While not wishing to be bound by theory, it is believed that thesecontrolled incremental stretches in the oxidation oven provide severalsignificant advantages. For example, greater amounts of cumulativestretching can help to further increase orientation within the PANfibers, and also help reduce the formation of flaw-inducing crystallitesin the fibers. When the stretches are conducted in a reactiveenvironment, such as the oxidation oven, these gains can be locked intothe fibers through chemical reactions which are occurring in the PANpolymeric chains. Stretching under non-oxidizing conditions may resultin loss of some of the gains due to heat relaxation and/or entropicrecovery. Additionally, strain rate distribution through controlledstretching of the fiber can also be applied in the low and/or hightemperature furnaces.

It has also been further found that distribution of strain over aplurality of passes in the oxidation oven also helps to improve theuniformity of oxidation and the rate at which oxidation occurs. Oneadvantage of controlled stretching is that tension load is applied inrelatively smaller cumulative stages, as opposed to a single large step.As a result, this permits the molecules to relax between subsequentstretches. Molecules that were initially drawn to undesirableconformations or orientations may then have a new chance to bereoriented in a more desirable manner, which can in turn help to improvethe tensile strength and the modulus of the resulting carbon fibers.Further, applying controlled stretching at the onset of stabilizationpermits thinning of the fibers, which in turn, helps to facilitate moreuniform oxidation at a faster rate than non-stretched fibers. As aresult, sources of inhomogeneity such as the formation of a skin-corestructure in the fibers can be reduced. This can help to further reducetension load gradients in the fibers and also improve the modulus andtensile strength of the resulting carbon fibers.

Additionally, it has been found that controlled stretching during theoxidizing step can result in reduced tension downstream of the oxidationoven(s). In one embodiment, incremental stretching in the oxidationstage can be used in conjunction with further stretching in the lowtemperature furnace. It is believed that more uniformly oxidized fibersare less affected by differential shear strain accumulation and cantherefore tolerate more tension and hence, can handle additional stretchduring the low temperature stage, which can provide additionalstructural gains, e.g., in molecular orientation, to be achieved in thelow temperature furnace. In one embodiment, the oxidized fiber can besubjected to a % stretch in the low temperature furnace that is betweenabout 1 and 40%, e.g., between about 1 and 30%, and 1 and 24%.

In one embodiment, multiple passes through the oxidation oven 20 can beused to distribute the tension loads over a plurality of oxidationpasses. For example, the speed at which each drive roll 30 is drivenwith respect to other drive rolls can be used to apply various tensionloads throughout a plurality of passes. In some embodiments, the speedof a subsequent drive roll can be reduced with respect a preceding driveroll, which results in a drop in tension in that pass. In some cases, adrop in tension can be used to permit shrinkage of the fiber duringoxidation. As noted above, stretching in a reactive environment may helplock-in mechanical structural gains that are obtained as a result of thecontrolled stretching. As a result, in some embodiments, the propertiesof fibers may first be enhanced by controlled stretching and then thefibers are permitted to shrink without losing the gains provided by thestretching process. This may permit the recapture of filament denier oran increase in weight per unit length that was lost in the previousstretches.

The desired amount of stretching in a given pass, the length of eachpass, the number of passes in an oxidation oven, and the residence timeof the fiber within the oxidation oven are dependent on the compositionof the precursor fibers and the desired properties of the carbon fibers.In one embodiment, the precursor fibers may make between about 2 and 20passes through the oxidation oven, and in particular between about 2 and10, such as between 4 and 8 passes through the oxidation oven. In someembodiments, the length of each pass may range between 4 and 40 feet.Generally, the residence time in the oxidation oven for each pass isbetween about 0.1 to 20 minutes, such as between about 1 to 12 minutesor 2 to 10 minutes.

In one embodiment, carbon fibers having improved strength and moduluscan be prepared by advancing the precursor through the oxidation oven inmultiple passes wherein the tension on the precursor fiber in at leasttwo or more of the passes is between about 100 to 1,000 mg/den.Generally, the maximum amount of % stretch to which the fiber issubjected to in a given pass is selected so that the strain rate isabout 10%/minute or less, and in particular less than about 5%/minuteper pass. Methods for determining the amount % stretch to apply in agiven pass for a given fiber are discussed in greater detail below. Thegains in mechanical properties attained through controlled stretchingare not restricted by the initial diameter, denier, or chemicalcomposition of the precursor fiber.

In one embodiment, carbon fibers having improved tensile strength andmodulus of elasticity can be prepared by subjecting a precursor fiberhaving a filament denier of about 1.5 dpf or less, and in particularless than 0.8 dpf to a cumulative % stretch that is between 5 and 100%and in particular between 15 and 60%. In yet another embodiment, theprecursor fiber is subject to a cumulative % stretch that is between 5and 70%, and more typically between 15 and 60%. In other embodiments,the precursor fiber is subjected to a plurality of controlled stretchesthat result in a 20 to 70% reduction in the fiber's diameter incomparison to the original diameter of the fiber prior to the oxidationstep. In still other embodiments, the precursor fiber has a reduction indiameter that is between 25 and 50%, and in particular, between 30 and45%. In one particularly useful embodiment, the method of stretching theprecursor fibers comprises subjecting the precursor fiber to a pluralityof controlled stretches in an oxidation oven wherein: a) the % stretchin a first pass is between 10 and 40% stretch; b) the % stretch in asecond pass is between 2 and 20% stretch; c) the % stretch in a thirdpass is between 2 and 16% stretch; and d) the % stretch in a fourth passis between 2 and 12% stretch. In a further embodiment, the oxidizedfiber can be subjected to a % stretch in a low temperature furnace thatis between 1 and 30% stretch. In one embodiment, carbon fibers preparedin accordance with invention can have tensile strength in excess of 950ksi, and in particular in excess of 1000 ksi, and a modulus ofelasticity in excess of 44 Msi, and in particular in excess of 50 Msi.

In one embodiment, the oxidized fibers can be carbonized,electrochemically surface treated, and sized with a protective coatingfor use in the preparation of structural composites, such as prepregs.In one embodiment, prepregs comprising the inventive carbon fibers canbe prepared that have an interlaminar or short beam shear strength thatis in excess of 19 ksi.

In a further aspect, the present invention is based on the recognitionthat improvements in modulus of elasticity and tensile strength ofcarbon fibers can be achieved by controlled stretching of the precursorfibers beyond limits previously taught in the prior art. In particular,it has been discovered that these improvements can be obtained bysubjecting the precursor fibers to a sufficient amount of stretch duringoxidation that causes the fibers to undergo one or more transitions intwo or more passes. In one embodiment, carbon fibers are prepared byadvancing a precursor fiber through an oxidizing atmosphere, such as anoxidation oven, in a plurality of passes in which the precursor fiber issubjected to controlled amount of stretching in two or more of thepasses so that precursor fiber undergoes at least two transitions ineach of the two or more passes. The transitions comprise a region ofinflection that is determinable from a plot of tension versus % stretchfor a given pass. FIG. 4 is an exemplary graphical plot of dynamic towtension versus % stretch in which a precursor fiber has undergone atleast three distinct transitions in a single pass through an oxidationoven. In the context of the invention, the term “dynamic tow tension”refers to the average tension measured in-line while running a towthrough a processing step. Specifically, in FIG. 4, dynamic tow tensionrefers to the steady-state tension in a particular pass experienced bytows of PAN fibers being continuously passed through the oxidation oven.The term “% stretch” is the same as defined by Equation 1 above.

From this plot, it can be seen that the fiber can undergo multipletransitions as it is subjected to controlled stretching in the oxidationoven. In the illustrated plot, the precursor fiber is depicted as havingundergone three transitions. As stretch increases from 0% to someinitial value, tension rises primarily because the constitutivemolecules stretched in prior steps tend to relax and contract. Thisrelaxation (also known as entropic recovery) is a function of appliedstretch because the molecules are more able to disentangle themselvesfrom their neighbors at higher stretches. Disentangled molecules are inturn able to carry more loads, but at increasingly higher % stretchestheir disentanglement and re-orientation becomes more difficult.Depending on parameters such as temperature, residence time, filamentdiameter, and extent of reaction, tension continues to rise untilfurther disentanglement and re-orientation become unfavorable. At thispoint, point A in FIG. 4, the tension begins to level off and a regionof inflection occurs as the precursor fibers are able to undergo morestretching without an increase in tension at the rate previouslyexperienced.

If higher stretches are applied, the molecules within the weakest amongthe more ordered (or pseudocrystalline) domains begin to be pulled apartand are forced to slide past each other. This requires overcoming thestrong nitrile dipole and/or hydrogen bonding interactions that holdthose regions together. Oxidative stabilization reactions can assist inthis regard. During oxidative stabilization, nitrile groups polymerizeto form ladder structures (see briefly, FIG. 1). These ladder structuresare oxidized very quickly, provided oxygen can diffuse fast enough toreach them. Otherwise, these ladder structures remain in the core of thefilaments. However, nitrile group polymerization (cyclization) consumesstrong dipoles, and hence promotes the sliding of molecules past eachother. This promotes additional disentanglement and re-orientation,along with a rise in effective tow tension.

As tension continues to rise, the cyclization frees more molecules forsliding and re-orientation. Oxidation of the ladder polymer, along withnitrile group crosslinking between neighboring chains (resulting in thegrowth of misoriented or angled ladder molecules), begin to contributesignificantly to the rise in tension. The tension begins to level offgradually as the stabilization process unfolds and the degree ofinterchain crosslinking increases. At a result, the precursor fibers areless able to sustain the same rate of stretching. Consequently, an areaof inflection occurs at point B in FIG. 4. Depending on the extent ofnitrile group polymerization, ladder polymer formation, and oxidativereactions, fibers stretched at relatively low strain rates can retainsignificant stretching potential. If the stabilization reaction islimited by the rate of oxygen diffusion, a skin-core structure develops,and the buildup of tension in the skin is larger than that in the core.This causes stress gradients that in the past may have limited theextent to which PAN fibers have been drawn during stabilization.However, as noted above, incremental stretching permits tension to beapplied in small cumulative stages so that the molecules can relaxbetween subsequent stretches. As a result, molecules that were initiallydrawn to undesirable conformations or orientations may then have a newchance to be reoriented in a more desirable manner.

Additionally, the higher % stretches at this stage may result in thinnerfilaments. Stretching the fibers during stabilization promotes oxygendiffusion into the core by increasing the effective filament surfacearea and reducing the diffusion distance. This in turn can lead to morehomogeneous fibers. However, the tension buildup in more oxidized fibersis larger than in less oxidized fibers. Hence, at sufficiently highstretches the tension begins to rise again.

Eventually as tension continues to rise past point B, the molecules maybegin to break due to the fracture of chains or tie molecules connectingthe remaining microfibrils. At this point, the shortest and/or moststressed (e.g., taut) among the tie molecules linking the remainingordered domains may begin to break, which may result in their loadburdens be transferred to longer and/or less stressed (e.g., slack) tiemolecules. Consequently, at this point the tension begins to level offagain and a third region of inflection occurs at point C on FIG. 4.Further transitions may be attributed to structural disruptions such asporosity development. Significant porosity development may be paralleledby a decrease in stabilized fiber density (ox-density) measured byhelium pycnometry.

In one embodiment, the occurrence of the transitions can serve as aguide for the % stretch that can be applied in each pass. Using thetransitions as a guide can help determine the maximum amount of stretchthat can be applied in a given pass so that the overall cumulativestretch can be increased. The exact positions and magnitudes of thetransitions can vary depending on processing parameters, such astemperature, residence times, heating rates, and the like, as well as onfiber properties, such as composition, filament diameter, structuralmorphology, and the like. Additionally, the positions of the transitionsmay also vary between subsequent passes in the oxidation oven. However,in practice the transitions can be tracked during the oxidizing stepfrom direct “dynamic tow tension” vs. “dynamic equilibrium stretch”readings. The number of transitions for a given precursor generallydepends on the target properties of the resulting carbon fiber products.In the case of PAN fiber stabilization, at least one transition per passis advantageous, and two or more transitions per pass in at least twopasses may be desirable. For example, when the fiber undergoes more thantwo transitions in a given pass, any negative impact in carbon fibermechanical properties due to taut tie molecule breakage (and subsequentradical reactions) or local porosity generation may still be offset bygains in molecular orientation and overall homogeneity due to improvedoxygen penetration and to the elimination of flaws. An example of how toidentify the transitions during oxidative stabilization is illustratedin Table 1 below.

Table 1 lists dynamic tow tension vs. stretch curve data for a 0.6 dpfPAN precursor fiber for a single pass through the oxidation oven. In theexample in Table 1, dynamic tow tensions were determined at stretchincrements of about 3% per data entry. In other embodiments, the stretchincrements may range from about 0.1 to 10%, or 2 to 6%. The secondcolumn lists the dynamic tensions measured after a stable tensionreading was reached. Column 3 lists the differences (Diff) betweentensions measured in two sequential stretch increments. For example,between 0 and 3% stretch, Diff=1092−795=297 g. Column 4 lists thedifferences (Der) between sequential differences calculated from Column3. For example, between 0 and 6% stretch, Der=219−297=−78 g. Note thatthe Der value is initially negative because the initial portion of thecurve represents a decreasing slope (i.e., it is a “concave downwards”curve). The terms “Diff” and “Der” are related to the first and secondderivatives of the curve, respectively. Hence, the second derivativeterm “Der” would be negative while the curve presents downwardsconcavity (or a decreasing slope). This “Der” term would also switch topositive if the slope of the curve began to increase (or the curveadopted an “upwards” concavity). The points of concavity reversal couldsimply be identified by recording dynamic tensions as a function ofstretch increments and noting the stretches at which the “Der” functionbecomes positive.

TABLE 1 Determination of Transition Points in Dynamic Tension vs.Stretch Curves. Stretch Tension Diff Der Tension Diff Der % (g) (g) (g)(mg/den) (mg/den) (mg/den) 0 795 — — 110 — — 3 1092 297 — 152 41.25 — 61311 219 −78 182 30.42 −10.83 9 1499 188 −31 208 26.11 −4.31 12 1571 72−116 218 20.00 −16.11 15 1715 144 72 238 10.00 10.00 18 1790 75 −69 24910.42 −9.58 21 1855 65 −10 258 9.03 −1.39 24 1909 54 −11 265 7.50 −1.5327 1957 48 −6 272 6.67 −0.83 30 1961 4 −44 272 0.56 −6.11 33 2018 57 53280 7.92 7.36 36 2068 50 −7 287 6.94 −0.97 39 2094 26 −24 291 3.61 −3.3342 2110 16 −10 293 2.22 −1.39 45 2120 10 −6 294 1.39 −0.83 48 2205 85 75306 11.81 10.42 51 2218 13 −72 308 1.81 −10.00 54 2245 27 14 312 3.751.94 57 2245 0 −27 312 0.00 −3.75 60 2256 11 11 313 1.53 1.53

In the example given in Table 1, three distinct transitions occur, whichcan be noted at 15%, 33%, and 48% stretch, respectively. Higher stretchtransitions become difficult to distinguish due to the smaller tensiondifferences involved. Columns 5-7 in Table 1 present the sameinformation as in Columns 2-4, but in units of mg/den (i.e., tensionnormalized by the tow's linear density, in order to facilitatecomparison with tows of different deniers). With reference to FIGS. 5and 6, the occurrence of the transitions can also be determined from aplot of the first and second derivatives, respectively.

As briefly noted above, the process described above in connection withTable 1 can be used to determine the % stretch to be applied in a givenpass. Once a desired or maximum limit is determined for the first %stretch, the above process is repeated for the second pass. This processcan then be repeated x number of times for x number of fiber passesthrough the oxidation oven. Generally, the process does not need to berepeated once the desired % stretch is determined for each pass for agiven precursor fiber at a given set of conditions.

From the foregoing discussion, it should be evident that the presentinvention provides a method for improving the tensile strength andmodulus of carbon fibers by controllably stretching the fibers in aplurality of passes through an oxidation oven. Carbon fibers prepared inaccordance with the invention are characterized by having both hightensile strength and high modulus of elasticity. As discussed above, theadvantageous properties of the carbon fibers can be obtained bysubjecting the precursor fibers to controlled stretching duringoxidative stabilization. In addition to improvements in tensile strengthand modulus of elasticity, carbon fibers prepared in accordance with theinvention have unique properties and structural features that have notbeen hitherto found in prior art carbon fibers. For instance, carbonfibers prepared in accordance with the invention may exhibit a highproportion of phase angles with values that are lower than average,increased surface roughness, increased average phase angle depths,higher resistivity values, and more uniform crystallinity distribution.

FIGS. 7A through 7F are surface phase images of various carbon fibersthat were obtained using Atomic Force Microscopy (AFM) in Tapping Mode.The surface phase images were obtained with a NanoScope IIa ScanningProbe Microscope manufactured by Digital Instruments Co. (Santa Barbara,Calif.). Tapping Mode AFM operates by scanning a tip attached to the endof an oscillating cantilever across the sample surface. The cantileveris oscillated at or around its resonance frequency with amplituderanging between 20-100 nm. The tip lightly taps onto the sample surfaceas it swings during x-y-z scanning. Tip-surface interactions cause thecantilever to oscillate with a characteristic phase pattern. Phasepatterns (in the form of phase contrast images) can be used todifferentiate between regions of strong and weak surface-tipinteractions. These regions may in turn be related to material featuressuch as, e.g., crystalline domains. In FIGS. 7A through 7F, individualfilaments of each carbon fiber sample (desized when needed) were mountedonto special AFM holders. Three surface regions (1 μm×1 μm) of eachfilament were scanned with an amplitude of 20 nm to generate the phaseangle images for the samples. Phase angle depth refers to the averagedistances between the peaks and valleys in the AFM images.

FIG. 7A is a surface phase image of a carbon fiber that was prepared bysubjecting a PAN precursor fiber to a 15% cumulative stretch in a singlepass in an oxidation oven. FIG. 7A corresponds to Comparative Example 1which can be found in Table 3 below. FIG. 7B is a surface phase image ofa carbon fiber that was prepared in general accordance with ComparativeExample 1, except that it was carbonized at a temperature in excess of1500° C. in order to increase modulus according to prior art methods.FIGS. 7C through 7F are surface phase images of carbon fibers that wereprepared in accordance with the invention. FIGS. 7C, 7D, 7E, and 7Fcorrespond to Examples 23, 35, 29, and 34, respectively, the processingdetails of which can be found in Table 6 below.

The lighter regions in the AFM images correspond to regions of higherphase angles on the carbon fibers and the darker regions correspond toregions of the carbon fibers of lower phase angles. All AFM images arepresented at the same scales to facilitate their direct comparison. InFIGS. 7A and 7B the AFM surface image of the carbon fibers prepared withconventional stretching methods are characterized by dark regions and/orlight regions. As shown in FIGS. 7C through 7F, the AFM images of thecarbon fibers prepared in accordance with the invention arecharacterized by the presence of a plurality of striations that extendacross the surface of the fibers. In one embodiment, the AFM images ofthe carbon fibers prepared in accordance with the invention arecharacterized by the presence of a plurality of striations of darkregions and a plurality of striations of light regions. For example, inFIGS. 7C and 7D the AFM image shows striations of both high phase angledomains and low phase angle domains that extend across the surface ofthe carbon fibers. These striations are even more apparent in FIGS. 7Eand 7F, which present images of carbon fibers within the same range ofmechanical properties but analyzed after having subjected them toin-line surface treatment and sizing steps as described in the ExamplesSection. Specifically, the images in FIGS. 7E and 7F also show aplurality of striations of low phase angle domains and striations ofhigh phase angle domains that extend across the surface of the carbonfibers. Referring back to FIGS. 7A and 7B and the AFM images of thecarbon fibers prepared in accordance with conventional stretchingmethods, the striations on the surface are less pronounced and coherentthan in the images of the present invention. It should be further notedthat, unlike dimensional surface topography images, phase angle depthimages are insensitive to surface curvature (or filament diameter)effects. Therefore, if the striations represent domains of differentdegrees of structural order, the AFM images indicate that the process ofthe invention is substantially altering the surface morphology of thecarbon fibers. The generation of ordered domains in the form of longstriations is consistent with increases in the parameter L_(a)(associated with average crystalline width or ribbon length) determinedby X-ray diffraction analysis, as discussed in greater detail below. InTable 2 below, it can be seen that the changes in the surface structuresof the fibers are accompanied by increases in both the tensile strengthand modulus of elasticity in the inventive carbon fibers. In comparison,the carbon fiber in FIG. 7A has a tensile strength and a modulus ofelasticity that is significantly less than the carbon fibers depicted inFIGS. 7C through 7F. Similarly, the carbon fiber in FIG. 7B has a highmodulus but its tensile strength falls below those of fibers of theinvention.

Additionally, the three-dimensional AFM images permit the surface of thecarbon fibers to be characterized on the basis of surface features suchas topographical roughness and phase angle depth distribution. In Table2 below, various parameters representing the phase angle depth androughness of the carbon fibers in the AFM images have beencharacterized.

TABLE 2 Summary of AFM Analysis for Carbon Fibers Depicted in FIGS.7A-7F Depth Section Tensile FIG. Example [nm] [L = 300 nm] RoughnessStrength Modulus No. No.^(a) Average Maximum Rq [°] Ra [°] Rq [°] Ra [°](ksi) (Msi) FIG. 7A Comp. 1 4.7 11 1.180 0.830 1.581 1.246 932 42.7 FIG.7B Comp. 8.9 23 3.702 2.649 3.198 2.509 732 55.1 AFM 1^(b) FIG. 7C 2315.1 32 4.499 3.363 3.956 3.038 1012 46 FIG. 7D 35 14.0 50 7.474 5.3918.198 5.925 1107 52.2 FIG. 7E 29^(c) 13.1 40 5.413 3.772 5.261 4.0131113 51.5 FIG. 7F 34^(c) 11.1 40 4.078 2.707 4.868 3.512 1058 47.3^(a)From Table 6 in the Example Section below. ^(b)Prepared inaccordance with Comparative Example 1, but carbonized at >1,500° C. (seeTable 4). ^(c)Includes surface treatment and sizing (see the ExampleSection below); samples were desized prior to scanning.

An analysis of the depth profiles for the carbon fibers in FIGS. 7Athrough 7F reveals that the carbon fibers prepared in accordance withthe invention have broader depth distribution functions (wider peaks)and a higher maximum depth (peak tails shifted towards increasingdepths). Generally, carbon fibers prepared in accordance with theinvention have an average phase angle depth greater than about 5 nm, andin particular greater than about 8 nm, and more particularly, greaterthan about 10 nm.

The surface roughness of the carbon fibers in the AFM images was alsocharacterized with Arithmetic Average Roughness (Ra) and Root MeanSquare Roughness (Rq) values.

Arithmetic Average Roughness (Ra) is calculated according to thefollowing equation:

$\begin{matrix}{{Ra} = \frac{\overset{n}{\sum\limits_{i}}{Z_{n}}}{n}} & (2)\end{matrix}$

Root Mean Square Roughness (Rq) is calculated according to the followingequation:

$\begin{matrix}{{Rq} = \sqrt{\frac{\overset{n}{\sum\limits_{i}}\left( Z_{n} \right)^{2}}{n\;}}} & (3)\end{matrix}$

In equations (1) and (2), Z_(i) values represent all the “n” deviationsin surface height measured from the mean data plane in the AFM image. Itshould be noted that two surfaces with different frequency of peaks canhave the same average Ra and/or Rq values. Nonetheless, surfaceroughness values provide quantitative information that complements theinformation provided by phase image depth analysis alone. The AFM imagesin FIGS. 7A through 7F were scanned over the entire 1 μm×1 μm region andthen Ra and Rq values were calculated using the above equations. Asection roughness of the AFM images in FIGS. 7A through 7F was alsoanalyzed by drawing an imaginary slice 300 nm long and perpendicular tothe orientation of the ordered phases.

The data in Table 2 show how the mechanical properties of representativecarbon fibers prepared in accordance with invention vary as a functionof parameter Ra. The higher the roughness in phase angle contrast, thehigher the mechanical properties of the corresponding carbon fibers. Forexample, Comparative Example 1 has a Ra value of about 1.2, a tensilestrength of 932 ksi, and a modulus of elasticity of 42.7 Msi. Similarly,the sample carbonized at a temperature exceeding 1500° C. (ComparativeAFM 1) has an overall Ra value of about 2.5, with a modulus exceeding 50Msi, but a tensile strength of only 732 ksi. The inventive carbon fibershaving Ra values greater than about 2 have a tensile strength andmodulus of elasticity that are significantly improved over thecomparative example carbon fibers. For instance, Example 23 has a Ravalue greater than 3 with a tensile strength of 1012 ksi and a modulusof elasticity 46 Msi. Example 29 having a Ra value greater than 4 has atensile strength of 1113 ksi and a modulus of elasticity 51.5 Msi.Generally, carbon fibers prepared in accordance with the invention mayhave a Ra value that is greater than about 2.0, and in particulargreater than 2.5, and more particularly greater than about 3.0.

The data in Table 2 also show how the mechanical properties ofrepresentative carbon fibers prepared in accordance with invention varyas a function of parameter Rq. As with parameter Ra, the higher theroughness in phase angle contrast, the higher the mechanical propertiesof the corresponding carbon fibers. Generally, carbon fibers prepared inaccordance with the invention may have an Rq value that is greater thanabout 2.0, and in particular greater than 3.0, and more particularlygreater than about 4.0.

From the data in Table 2 and the AFM images in FIGS. 7C through 7F, itcan be seen that the inventive process of preparing the fibers resultsin fiber having increased tensile strength and modulus of elasticity,and that this increase in strength and modulus is accompanied bystructural changes that are occurring in the fibers, e.g., theappearance of continuous striations on the surface of the fibers,increases in phase angle depths, and increases in roughness on thesurface of the fibers.

In addition to the surface features discussed above, carbon fibersprepared in accordance with the invention also have unique crystallitedimensions (L_(a) and L_(c)). Table 3 below contains crystallite datafor carbon fibers prepared in accordance with the invention. The data inTable 3 were obtained by subjecting various carbon fibers to X-raydiffraction analysis. The measurements were carried out in reflectionmode with the fiber strands aligned perpendicular to the diffractionplane on a wide angle powder diffractometer using CuKα radiation with adiffracted beam monochromator. Determination of L_(a) and L_(c) used therelationship of L_(a)=1.84×λ/(B×cos(θ)) and of L_(c)=0.89×λ/(B×cos(θ)),respectively, wherein λ is 1.54

, B is the peak width in (° 2 θ), and θ is the diffraction angle.

TABLE 3 Carbon Fiber X-ray Diffraction Data Carbonization ExampleTemperature Strength Modulus Diameter Density L_(c) L_(a) No. (° C.)(ksi) (Msi) (μm) (g/cm³) (

) (

) Comparative 1,315 982 44.9 4.3 1.809 16.8 43.8 Example 3 60 1,315 97747.1 3.5 1.815 16.4 48.7 29 1,315 1113 51.5 3.6 1.809 16.4 56.5

Generally, the parameter L_(a) is associated with the “crystallitewidth” or “ribbon length”, and parameter L_(c) is associated with“crystallite height.” In one embodiment, carbon fibers prepared inaccordance with the invention have L_(a) values that are about 4 nm orgreater, and in particular greater than about 4.5 nm, and in someembodiments greater than 5.0 nm. The data in Table 4 suggest that carbonfibers made in accordance with the invention tend to have larger L_(a)values than conventional PAN-based carbon fibers made at comparabletemperatures. The carbon fibers exemplified in Table 3 were prepared atcarbonization temperatures of about 1,315° C. and have L_(a) values thatare higher than what would normally be expected at these relatively lowcarbonization temperatures. Conventional PAN-based fibers havingcomparable L_(a) values are typically prepared with conventionaltechniques that heat the fibers above 1,750° C., and even above 2,000°C., e.g., high enough to induce graphitization in the carbon fibers. Thecarbon fibers exemplified in Table 3 were prepared at carbonizationtemperatures of about 1,315° C. and have L_(a) values that are higherthan what would normally be expected from PAN-based precursors processedat these relatively low carbonization temperatures. On the other hand,mesophase pitch-based precursors are known to exhibit high L_(a) valuesat low carbonization temperatures. However, the higher L_(a) valuesexhibited by mesophase pitch-based fibers are accompanied by lowertensile strength values. As noted above, the high L_(a) values areconsistent with the distinct striations noted on pertinent AFM phaseimages, and again point to the unique nature of carbon fibers preparedaccording to the invention. Carbon fibers prepared according to thepresent invention can provide a favorable balance between the need forhighly oriented extended ribbons (to enhance modulus) along withcrystalline domains of relatively small dimensions (to enhancestrength).

In another embodiment, carbon fibers prepared in accordance with theinvention can also be characterized by their resistivity. Fiber towelectrical resistivities were determined using a commercially availabledigital multimeter (GW Instek Model GDM-8055). The meter was connectedto a fiber support frame designed to align tows for measuring theelectrical resistance (R) across multiple 0.20 m segments of acontinuous fiber. Average R values from 6-10 segments per sample wereused to calculate electrical resistivities (r) using the followingequation:

r=5RW/ρ  (4)

where R (in Ohms) is the average measured electrical resistance, W (ing/m) is the fiber weight per unit length, WPUL, as listed in Tables 6-9,ρ (in g/cm³) is the fiber density (also listed in Tables 6-9); and r iscalculated in units of μΩm.

As shown in Table 4, carbon fibers prepared in accordance with theinvention have a relatively higher resistivity than what would normallybe expected for fibers having similar modulus of elasticity.Conventional art methods to increase carbon fiber modulus (such ascarbonizing the fibers above 1,500° C.) are generally accompanied by adecrease in electrical resistivity. While conventional processingmethods lead to the expected decrease in resistivity with increasingmodulus, the resistivity of the inventive carbon fibers remainedrelativity high (above 13 μΩm, and in some embodiments above 14 μΩm) atmodulus of elasticity above about 50 Msi.

TABLE 4 Resistivity Data of Representative Carbon Fibers CarbonizationExample Temperature Strength Modulus Diameter Density Resistivity No. (°C.) (ksi) (Msi) (μm) (g/cm³) (μΩm) Comparative 1 1,315 932 42.7 4.31.797 13.0 31 1,315 893 43.5 3.5 1.806 14.3 32 1,315 972 47.8 3.5 1.83214.3 33 1,315 1061 47.8 3.5 1.815 14.0 35 1,315 1107 52.2 3.6 1.806 14.2Comparative >1500° C. 500 64.1 4.5 1.880 8.7 Resistivity Example 1Comparative >1500° C. 732 55.1 5.0 1.768 11.3 Resistivity Example 2

In the Examples in Table 4 above, Comparative Example 1, Examples 31through 33, and Example 35 were prepared according to the processdescribed in Table 6 below. Comparative Resistivity Examples 1 and 2represent conventional high modulus fibers prepared in generalaccordance with the process described for Comparative Example 1 in Table6, except that they were carbonized at temperatures in excess of 1,500°C.

Further, carbon fibers prepared in accordance with the invention alsohave a more uniform distribution of crystallinity in a single filament.In one embodiment, carbon fibers of the present invention arecharacterized by having a difference in crystallinity “RD” between theinner and outer layers of each single filament that is less than about0.05, and in particular less than about 0.015. RD can be determinedusing RAMAN analysis. An exemplary method of determining RD is describedin U.S. Pat. No. 6,428,892, the contents of which are herebyincorporated to the extent that they are consistent with the teachingsherein.

Carbon fibers having small or little structural difference between theinner and outer layers exhibit small differences in “RD” between theinner and outer layers. In one embodiment, the carbon fibers of thepresent invention exhibit a difference “RD” between the inner and outerfrom about 0.01 to 0.05, and more particularly between 0.01 and 0.025.

The crystalline uniformity of the inventive carbon fibers can also becharacterized in terms of radial uniformity (RU) of the carbon fiber. Asdefined in this disclosure, the Radial Uniformity parameter “RU”provides a relative measure of strain distribution differences betweenthe outer and inner layers of carbon fiber filaments. The derivation andthe significance of RU values are outlined below.

Radial uniformity was evaluated with a Jobin-Yvon “LabRAM” RamanSpectrometer, equipped with a 1,800 gr/mm grating and excited by an Ar⁺ion laser beam (514.5 nm wavelength) focused with a ×100 objective lens.Sample filaments were mounted with their cross-sections orientedperpendicular to the direction of the laser beam. After securing themounts against vibration sources, the laser beam was independentlyfocused on two radial locations within selected filament cross-sections:(a) at the center of the filament, and (b) near the external surface ofthe filament. Inelastically scattered radiation intensity I_(c) orI_(s), respectively) was measured at high spectral resolution (atwavenumber increments of ˜1 cm⁻¹) within lower and upper wavenumberboundaries determined by the shape of the Raman scattering peaksattributable to ordered and disordered carbon structures in the sample.Typical lower and upper boundaries for these peaks were about 700 cm⁻¹or less and 2,220 cm⁻¹ or more, respectively. It should be noted thatsince the laser spot at the sample has a diameter of 1 micron,additional refinements may be required to extend this technique tofilament diameters below 3 microns.

Radial Uniformity (RU) values are derived by noting that Raman peaksfrom carbonaceous materials in general, and carbon fibers in particular,represent the combined response of a multitude of bonds stressed todifferent degrees. Hence, the positions and shapes of the Raman peaksvary in relation to the amount of strain experienced by the bondscontained within the region probed. Raman spectra of carbon fibersgenerally exhibit two broad peaks: one with maximum intensity at about1,560-1,600 cm⁻¹, associated with ordered structures akin to graphite (Gband), and another one with maximum intensity at about 1,330-1,370 cm⁻¹,associated with disordered structures (D band). These two broad peaksoverlap to an extent determined by their relative distribution of bondstresses. Parameter RU quantifies this extent on the basis of Ramanintensities at frequencies representative of the average degree ofstrain within ordered (Y_(g)) and disordered (Y_(d)) regions of a givensample. Noting that the difference between Raman intensity ratiosY_(d)/Y_(g) measured at the outer (or skin) layers versus the inner (orcore (layers) is inversely proportional to radial uniformity, it followsthat parameter RU can be calculated as:

RU=((I _(s,g) −I ^(b) _(s,g))(I _(c,g) −I ^(b) _(c,g)))/((I _(s,d) −I^(b) _(s,d))(I _(c,g) −I ^(b) _(c,g))−(I _(s,g) −I ^(b) _(s,g))(I _(c,d)−I ^(b) _(c,d)))  (5)

In Equation 5, Raman intensities I are taken directly from raw data (inorder to avoid biases due to curve-fitted baseline corrections),subscripts s and c refer to the skin and core regions of a filamentcross-section, respectively, subscripts g and d refer to the ordered anddisordered regions as defined above, respectively, and superscript brefers to baseline values derived from the raw data plots. For thepurposes of this application, intensities I_(g) and I_(d) are associatedwith intensity values generally within 1,560-1,600 cm⁻¹ and 1,460-1,500cm⁻¹, respectively. RU values calculated according to Equation 5 aredirectly proportional to radial uniformity, and are very sensitive tosmall differences in radial strain distributions, as provided by thefibers of this invention.

Representative examples of RU values for carbon fibers of the inventionare detailed in Table 5 below. The examples in Table 5 correspond to theexamples in Table 6 below. Generally, carbon fibers in accordance withthe invention have relatively high RU values. In one embodiment, thecarbon fibers have an RU value of about 50 or greater, and in particularabout 75 or greater. In some embodiments, the carbon fibers may have anRU value of about 100 or greater. In one embodiment, the inventioncomprises carbon fibers having a tensile strength of at least 975 ksi, amodulus of elasticity of at least 46 Msi and an RU value of about 50 orgreater.

TABLE 5 Radial Uniformity of Representative Carbon Fibers ExampleStrength Modulus Diameter Density No. [ksi] [Msi] (μm) (g/cm³) RUComparative 932 42.7 4.3 1.797 22 Example 1 Comparative 982 44.9 4.31.809 29 Example 3 23 1012 46.0 3.6 1.805 91 29 1113 51.5 3.6 1.809 100060 977 47.1 3.5 1.815 59

The crystallinity difference between the inner and outer layers ofsingle fiber is generally believed to be a function of the ability ofoxygen to diffuse into the fibers during stabilization. As discussedabove, the present invention provides for better control and moreuniform oxidation of the PAN fibers during oxidation, and hence resultsin a more uniform distribution of crystallinity between the inner andouter portions of the carbon fibers. Generally, the crystallinitydifference between the inner and outer layers increases as the thicknessof the fibers increases. In the present invention, controlled stretchinghelps to thin the individual filaments, which in turn, permits moreuniform, diffusion of oxygen into the fibers. As a result, thedistribution of crystallinity in the fibers is also more uniform.According to the invention, carbon fibers having reduced areas oftensile stress concentration can be produced. Higher crystallinity nearthe outer layers of the carbon fibers may result in points of hightensile stress that can result in breakage or damage to the carbonfibers, which results in carbon fibers of lower tensile strength.

Carbon fibers prepared according to the invention can also provide avariety of strain values when loaded to failure. In particular,strain-to-failure values approaching 2.5% have been measured from fibersthat would exhibit about 2% strain-to-failure when made following priorart methods.

Carbon fibers made according to the present invention also providedistinct benefits for composite manufacture. Since the inventionprovides a method of obtaining carbon fibers having relatively highmodulus at relatively low carbonization temperatures, the surfaces ofthe carbon fibers can remain reactive. This fact enables milder surfacetreatments and a more effective interaction with composite resins forimproved mechanical property translation. Low carbonization temperaturescan also be beneficial towards enhancing compressive strength. Inaddition, the present invention can be used to prepare carbon fibershaving relatively small diameters, which can provide benefits in termsof (a) reductions in filament rigidity for improved tow moldability; and(b) the increased specific surface area for fiber-resin contact. Thisincreased specific surface area, coupled with an enhanced phase angleroughness as revealed by AFM analysis, may help improve chemical bondingmechanical interlocking between the fibers and the resin.

The following Examples are provided for illustrating aspects of theinvention and should not be construed as limiting the invention. Unlessotherwise indicated all modulus of elasticity measurements cited in theExamples were made according to ASTM D 4018, as described in more detailin U.S. Pat. No. 5,004,590, the contents of which are herebyincorporated by reference. Fiber modulus values refer to tensile chordmoduli of resin-impregnated tow strands determined between lower andupper strain limits of 0.1% and 0.6%, respectively. Moreover, tensilestrengths were measured according to ASTM D 4018, as described in moredetail in U.S. Pat. No. 5,004,590, the contents of which are herebyincorporated by reference.

EXAMPLES

In the examples listed in Tables 6-8, polyacrylonitrile (PAN) precursorfibers were made from a copolymer consisting of 98 mol % acrylonitrileand 2 mol % methacrylic acid by an air gap wet spinning process. Thestarting copolymer had an intrinsic viscosity of about 2.0 decilitersper gram when determined using concentrated sodium thiocyanate as thesolvent. In the examples in Table 9, PAN precursor fibers were made froma terpolymer consisting of 93.0 mol % acrylonitrile, 1.5 mol % itaconicacid and 5.5 mol % methyl acrylate by a wet spinning process. In all theexamples, the spinning and coagulant solutions were based on aqueoussodium thiocyanate. The fibers were stretched during spinning so thattheir length in tow form became up to about six times greater aftersteam stretching compared to their length after extrusion from thespinnerettes. The dried precursor fibers contained about 1 wt. % of asilicone-based finish oil, and had filament deniers of 0.60 (Table 6),0.8 (Table 7), 1.33 (Table 8) and 1.53 (Table 9) dpf. Tables 6-9 listresults for fibers with filament counts between 6,000 and 24,000filaments per tow.

The precursor fibers were stabilized by passing them through a series offorced air convection ovens generally held at increasing oxidationtemperatures (exceptions are noted in the footnotes in Tables 6-9). Thefirst oven was configured to enable controlled stretching or shrinkagein multiple passes, as illustrated in FIG. 2. In the following examples,four passes with external roll assemblies were employed. Each rollassembly included its own individual motor and multiple rolls coupled bygears in order to increase the contact between the fiber and the surfaceof the rolls. The first driven roll assembly advanced the fibers throughthe oven and onto an idler roll connected to a load cell. The fibersmade a 180 degree turn at the load cell idler roll and returned to theoven towards a second driven roll assembly. The second roll assembly wasgenerally driven at a speed higher than the first assembly, so theeffective % stretch in the first pass was correspondingly higher. Speedsin subsequent roll assemblies were adjusted so that the effective %stretches experienced by the fibers in the subsequent passes were alsovaried.

The information provided in Tables 6-9 may be best understood byreferring to Example 35 in Table 6 for illustration. In Example 35 thesecond roll assembly was driven at a speed 20% higher than the firstassembly, so the effective % stretch in the first pass Table 6) was20.0%. Speeds in subsequent roll assemblies were adjusted so that theeffective % stretches experienced by the fibers in three additionalpasses were 13.3% (#2), 10.3% (#3) and 6.70% (#4). Hence, the overallcumulative % stretch (OX-Str) experienced by this fiber within the firstoxidation oven was 60.0%. During controlled stretching the firstoxidation oven was maintained at a temperature (Temp Ox.) of 223° C.Following the controlled stretching of the fibers, the fibers wereadvanced at 0% additional stretch through three more oxidation ovensmaintained at incrementally higher temperatures up to 246° C. Theoverall residence time in all four of the oxidation ovens was less than90 minutes. Other Examples cover a broad range of experimentalparameters, as indicated in footnotes to the Tables and discussed inmore detail below.

The stabilized fibers from Example 35 were then advanced through low andhigh temperature furnaces having average temperatures of 523° C. and1,315° C., respectively. In some of the following examples, the hightemperature furnace was operated at higher temperatures, e.g., 1,500° C.or higher. In Example 35 the inlet and outlet speeds of the fibersthrough these carbonization furnaces were set to deliver effective %stretches in the low and high temperature furnaces of 21.1% and −3.5%(shrinkage), respectively, by means of isolation rollers. In otherExamples, the degrees of stretch in the low temperature furnace and ofshrinkage in the high temperature furnace were modified as listed inTables 6-9. Unless noted otherwise, the total residence time in thecarbonization zones was about one tenth of that in the oxidativestabilization portions of the process.

After completion of the carbonization step in Example 35, the fiber wassurface treated by subjecting it to anodic oxidation in an ammoniumbicarbonate bath with a charge of 90 Coulombs per square meter. Thesurface treated fiber was sized with an epoxy-compatible sizing agent,and was dried and wound in spools prior to testing or further processingto form a composite. In the case of Example 35, sufficient quantities ofcarbon fiber were made to produce a 6-inch resin-impregnated prepregtape and test its properties. The determination of interlaminar or shortbeam shear strength by a Laminate test described in U.S. Pat. No.5,004,590, which has previously been incorporated by reference,indicated that the composite had a short beam shear strength of 19.0ksi.

The effects and resulting fiber properties of controlled stretching inthe oxidation oven were investigated for four different types of fibers.The following examples were prepared on a research pilot line over anextended period. During that period, mechanical changes were made toimprove the performance of the line and in particular to reduce tensionvariability arising from various sources. Reduction in tensionvariability to minimum levels can be achieved by methods known in thefield of fiber technology, including the use of slip-free rolls andvibration-damping devices. Some experimental variability in the resultsis to be expected, with typical coefficients of variation being within2-3%. However, the apparent variability in some of the examples in theTables is more a reflection of their being listed in order of increasingoxidation stretch severity, as opposed to increasing chronological orderor experimental test set. Pertinent footnotes in each Table account forthese and other deviations from the standard norm.

The results are summarized in the following Tables.

-   -   1) Table 6: The Examples in Table 6 were prepared using PAN        precursor fibers (AN:MAA) having a denier of 0.60 dpf and a        fiber diameter of 8.5 μm.    -   2) Table 7: The Examples in Table 7 were prepared using PAN        precursor fibers (AN:MAA) having a denier of 0.80 dpf and a        fiber diameter of 9.8 μm.    -   3) Table 8: The Examples in Table 8 were prepared using PAN        precursor fibers (AN:MAA) having a denier of 1.33 dpf and a        fiber diameter of 12.6 μm.    -   4) Table 9: The Examples in Table 9 were prepared using PAN        precursor fibers (AN:IA:MA) having a denier of 1.53 dpf and a        fiber diameter of 13.5 μm.

In Tables 6 through 9 the following column headings are defined asfollows:

a) “% Stretch in OX-Pass #” refers to the % stretch between successivedrive rolls;b) “OX-Str.(%)” refers to the cumulative % stretch of the fiber in theoxidation oven;c) “Temp. OX (° C.)” refers to the temperature of the oxidation oven;d) “TR-Str. (%)” refers to % stretch in the low temperature furnace(also referred to as the tar remover furnace);e) “Temp.TR (° C.)” refers to the temperature in the low temperaturefurnace;f) “C2-Str. (%)” refers to the % stretch in the high temperature furnace(also referred to as the carbonization furnace);g) “HTT (° C.)” refers to the temperature in the high temperaturefurnace;h) “WPUL” refers to the weight per unit length of the carbon fiber;i) “Density” refers to the density of the carbon fiber;j) “Diameter” refers to the average diameter of the carbon fiberfilaments, calculated from the fiber WPUL and density; andk) “Strength” and “Modulus” are as defined above.

TABLE 6 Processing Conditions and Property Data for Carbon Fibersprepared from PAN precursor fibers having a denier of 0.60 dpf Example %Stretch in OX-Pass # OX-Str. Temp. TR-Str. Temp. No. 1 2 3 4 (%) OX (°C.) (%) TR (° C.) Comparative 1 15 0 0 0 15 223 12 523 Comparative2^(a,b) 15 0 0 0 15 223 12 523 Comparative 3^(b) 15 0 0 0 15 223 12 523Comparative 4^(b) 15 0 0 0 15 223 12 523 Comparative 5^(b,c) 15 0 0 0 15223 12 523 Comparative 6 15 0 0 0 15 223 12 523 Comparative 7 20 0 0 020 223 12 523 Comparative 8 48 0 0 0 48 223 12 523 Comparative 9 48 0 00 48 223 24 523 Comparative 10 60 0 0 0 60 223 24 523  1 0 0 0 15 15 22312 523  2 3.6 3.6 3.6 3.6 15 223 12 523  3 5 5 4.3 0 15 223 12 523  4 104.6 0 0 15 223 12 523  5 20 −4.2 0 0 15 223 12 523  6 5 5 4.3 4.3 20 22312 523  7 5 5.5 6 6.5 25 223 12 523  8 5 7 8 9 32.3 223 12 523  9 5 9 1011.2 40 223 12 523 10 5 9 10 11.2 40 223 16 523 11 5 9 10 11.2 40 223 16523 12 5 9 10 11.2 40 223 16 523 13 5 9 10 11.2 40 223 16 523 14 5 9 1011.2 40 223 24 523 15 10 9.2 8.2 7.7 40 223 12 523 16 15 8.7 6.4 5.3 40223 12 523 17 15 8.7 6.4 5.3 40 223 16 523 18 15 8.7 6.4 5.3 40 223 24523 19 20 15 3 2 45 223 21 523 20 20 15 3 2 45 223 12 523 21 12.5 11.110 9.1 50 223 12 523 22 15 11.3 9.4 7.1 50 223 12 523 23 15 11.3 9.4 7.150 223 24 523 24 15 11.3 9.4 7.1 50 223 24 523 25 25 20 0 0 50 223 18523 26 25 20 0 0 50 223 24 523 27^(h) 20 13.3 10.3 6.7 60 223 12 523 2820 13.3 10.3 6.7 60 223 18 523 29^(g) 20 13.3 10.3 6.7 60 223 21 52330^(h) 20 13.3 10.3 6.7 60 223 21 523 31^(d) 20 13.3 10.3 6.7 60 223 21523 32^(e) 20 13.3 10.3 6.7 60 223 21 523 33^(f) 20 13.3 10.3 6.7 60 22321 523 34^(g) 20 13.3 10.3 6.7 60 223 21 523 35^(b,g) 20 13.3 10.3 6.760 223 21 523 36^(h) 20 13.3 10.3 6.7 60 223 24 523 37 20 13.3 10.3 6.760 223 24 523 38^(b) 20 13.3 10.3 6.7 60 223 24 523 39 25 20 4 2.6 60223 24 523 40 25 20 4 2.6 60 223 24 523 41 25 24 2 1.2 60 223 24 523 4225 24 2 1.2 60 223 21 523 43 25 24 2 1.2 60 223 21 523 44 25 24 2 1.2 60223 21 523 45 25 24 2 1.2 60 223 21 523 46 25 20 4 2.6 60 223 21 523 4725 20 4 2.6 60 223 21 523 48 25 20 4 2.6 60 223 21 523 49 25 20 4 2.6 60223 21 523 50 25 20 4 2.6 60 223 21 523 51 25 20 4 2.6 60 223 21 523 5225 20 4 2.6 60 223 21 523 53 25 20 4 2.6 60 223 21 523 54 25 20 4 2.6 60223 12 523 55 25 20 4 2.6 60 223 24 523 56 25 20 4 2.6 60 223 24 523 5725 20 4 2.6 60 223 24 523 58 25 25 4.3 1.2 65 223 24 523 59 25 20 9.36.7 70 223 18 523 60 30 23 4 2 70 223 18 523 61 30 23 4 2 70 223 24 523Example C2-Str. HTT WPUL Density Diameter Strength Modulus No. (%) (°C.) (g/m) (g/cm³) (μm) (ksi) (Msi) Comparative 1 −3.5 1315 0.317 1.7974.3 932 42.7 Comparative 2^(a,b) −3.5 1315 0.319 1.829 4.3 965 41.1Comparative 3^(b) −3.5 1315 0.319 1.809 4.3 982 44.9 Comparative 4^(b)−3.5 1315 0.319 1.829 4.3 1047 40.3 Comparative 5^(b,c) −3.5 1315 0.1551.802 4.3 1079 41.5 Comparative 6 −3.5 1315 0.320 1.806 4.3 975 41.2Comparative 7 −3.5 1315 0.304 1.800 4.2 991 40.9 Comparative 8 −3.5 13150.244 1.808 3.8 979 44.7 Comparative 9 −3.5 1315 0.230 1.808 3.7 93042.4 Comparative 10 −3.5 1315 0.225 1.812 3.6 882 47.4  1 −3.5 13150.323 1.799 4.4 1026 39.9  2 −3.5 1315 0.328 1.799 4.4 939 38.8  3 −3.51315 0.321 1.808 4.3 956 41.8  4 −3.5 1315 0.318 1.808 4.3 941 40.5  5−3.5 1315 0.319 1.803 4.3 995 41.0  6 −3.5 1315 0.308 1.807 4.3 914 42.2 7 −3.5 1315 0.297 1.806 4.2 903 40.7  8 −3.5 1315 0.281 1.809 4.1 92139.9  9 −3.5 1315 0.266 1.803 4.0 1006 43.3 10 −3.5 1315 0.245 1.815 3.8966 44.8 11 −3.5 1425 0.255 1.777 3.9 953 45.7 12 −3.4 1425 0.245 1.8103.8 980 44.5 13 −3.2 1425 0.221 1.819 3.6 1078 48.9 14 −3.5 1315 0.2201.795 3.6 1166 48.3 15 −3.5 1315 0.262 1.803 3.9 994 43.4 16 −3.5 13150.261 1.806 3.9 1008 44.3 17 −3.5 1425 0.250 1.777 3.9 965 45.7 18 −3.41425 0.232 1.780 3.7 955 47.3 19 −3.5 1315 0.348 1.798 4.5 947 43.6 20−3.5 1315 0.349 1.800 4.5 1040 47.9 21 −3.5 1315 0.244 1.798 3.8 96343.9 22 −3.5 1315 0.246 1.803 3.8 997 44.6 23 −3.5 1315 0.220 1.805 3.61012 46.0 24 −3.5 1425 0.219 1.780 3.6 999 47.1 25 −3.5 1315 0.229 1.8083.7 1016 45.2 26 −3.5 1315 0.218 1.809 3.6 972 46.0 27^(h) −3.5 13150.228 1.801 3.7 970 44.6 28 −3.5 1315 0.218 1.804 3.6 979 45.5 29^(g)−3.5 1315 0.218 1.809 3.6 1113 51.5 30^(h) −3.5 1315 0.212 1.807 3.51008 46.8 31^(d) −3.5 1315 0.211 1.806 3.5 893 43.5 32^(e) −3.5 13150.210 1.832 3.5 1061 47.8 33^(f) −3.5 1315 0.213 1.815 3.5 972 47.834^(g) −3.5 1315 0.212 1.817 3.5 1058 47.3 35^(b,g) −3.5 1315 0.2181.806 3.6 1107 52.2 36^(h) −3.5 1315 0.206 1.807 3.5 1014 45.9 37 −3.51315 0.191 1.809 3.4 1073 50.0 38^(b) −3.5 1315 0.206 1.801 3.5 114153.3 39 −3.5 1315 0.204 1.808 3.5 984 46.7 40 −3.5 1425 0.204 1.782 3.5963 46.9 41 −3.5 1315 0.205 1.810 3.5 980 46.5 42 −3.5 1315 0.211 1.8073.5 960 45.2 43 −3.5 1425 0.209 1.784 3.5 957 46.6 44 −3.5 1525 0.2061.765 3.5 926 47.9 45 −3.5 1695 0.203 1.749 3.5 842 50.0 46 −3.5 13150.209 1.811 3.5 953 45.0 47 −3.5 1420 0.206 1.782 3.5 915 45.5 48 −3.51520 0.203 1.765 3.5 910 46.7 49 −3.5 1610 0.202 1.757 3.5 878 49.0 50−3.5 1690 0.202 1.761 3.5 865 50.3 51 −3.5 1790 0.201 1.746 3.5 836 51.952 −3.5 1890 0.199 1.759 3.5 861 54.9 53 −3.5 1980 0.199 1.764 3.5 85658.0 54 −3.5 1315 0.227 1.805 3.7 974 44.3 55 −3.5 1315 0.206 1.817 3.5977 48.3 56 −3.3 1420 0.201 1.786 3.5 951 48.3 57 −3.3 1690 0.196 1.7483.5 822 51.7 58 −3.5 1315 0.199 1.809 3.4 974 45.9 59 −3.5 1315 0.1931.806 3.4 1010 47.2 60 −3.5 1315 0.204 1.815 3.5 977 47.1 61 −3.5 13150.200 1.812 3.4 1002 49.0 ^(a)Improved equipment components. ^(b)Reducedtension variability. ^(c)6,000 (vs. 12,000) filaments per tow. ^(d)Runexcluding surface treatment and sizing application (blank run).^(e)Including surface treatment but no sizing. ^(f)Including sizing butunstable emulsion. ^(g)Including surface treatment and sizing.^(h)Relatively high tension variability during carbonization.

TABLE 7 Processing Conditions and Property Data for Carbon Fibersprepared from PAN precursor fibers having a denier of 0.80 dpf % Stretchin Temp. Example OX-Pass # OX-Str. OX. TR-Str. Temp. C2-Str. HTT WPULDensity Diameter Strength Modulus No. 1 2 3 4 (%) (° C.) (%) TR (° C.)(%) (° C.) (g/m) (g/cm³) (μm) (ksi) (Msi) Comparative 0 0 0 0 0 223 0523 −4.1 1315 0.570 1.776 5.84 762 38.1 2.1 Comparative 9.1 0 0 0 9.1223 12 523 −3.5 1315 0.460 1.784 5.23 844 41.2 2.2 Comparative 24 0 0 024 223 24 523 −3.5 1315 0.357 1.805 4.58 879 41.7 2.3 Comparative 30 0 00 30 223 24 523 −3.5 1315 0.342 1.802 4.49 859 44.8 2.4 Comparative 50 00 0 50 223 24 523 −3.5 1315 0.295 1.803 4.17 880 44.5 2.5 2.6 9.1 10 0 020 223 12 523 −3.5 1315 0.408 1.794 4.91 872 41.9 2.7 9.1 10 6.7 0 28223 12 523 −3.5 1315 0.383 1.793 4.76 898 41.2 2.8 9.1 10 6.7 5.5 35 22312 523 −3.5 1315 0.364 1.793 4.64 894 42.4 2.9 20 20 0 0 44 223 24 523−3.5 1315 0.310 1.798 4.28 915 41.5 2.10^(a) 20 20 0 0 44 223 24 523−3.5 1315 0.308 1.796 4.26 892 41.2 2.11^(b) 20 20 0 0 44 223 24 523−3.5 1315 0.303 1.805 4.22 864 40.5 2.12^(b) 20 20 0 0 44 223 30 523−3.5 1315 0.287 1.805 4.11 839 44.5 2.13^(b,c) 20 20 0 0 44 223 24 523−3.5 1315 0.301 1.794 4.22 892 43.3 2.14^(b,c) 20 20 0 0 44 223 30 523−3.5 1315 0.285 1.814 4.08 858 45.3 2.15^(b,c) 20 20 0 0 44 223 33 523−3.5 1315 0.288 1.807 4.11 791 45.4 2.16 15 15 7 6 50 210 24 523 −3.51315 0.301 1.796 4.22 877 44.5 2.17 15 15 7 6 50 223 24 523 −3.5 13150.303 1.795 4.23 906 44.9 2.18 15 15 7 6 50 236 24 523 −3.5 1315 0.3031.792 4.24 893 45.0 2.19 15 15 7 6 50 249 24 523 −3.5 1315 0.310 1.7884.29 689 44.2 2.20^(d) 15 15 7 6 50 236 24 523 −3.5 1315 0.306 1.7974.25 888 44.7 2.21 20 20 10 −5.3 50 223 24 523 −3.5 1315 0.300 1.7994.20 884 44.7 2.22 20 25 10 −9.1 50 223 24 523 −3.5 1315 0.299 1.7974.20 870 45.5 2.23^(c) 20 20 10 −5.3 50 223 24 523 −3.5 1315 0.300 1.8004.20 894 47.0 2.24^(e) 20 20 10 −5.3 50 223 24 523 −3.5 1315 0.402 1.6185.14 198 8.4 2.25 24 24 0 0 54 223 24 523 −3.5 1315 0.290 1.805 4.13 88343.2 2.26 24 24 0 0 54 223 28 523 −3.5 1315 0.283 1.794 4.09 872 43.52.27 25 28 0 0 60 223 24 523 −3.5 1315 0.271 1.802 3.99 901 44.52.28^(b) 25 28 0 0 60 223 24 523 −3.5 1315 0.270 1.807 3.98 820 44.82.29^(b,c) 25 28 0 0 60 223 24 523 −3.5 1315 0.273 1.807 4.00 795 46.82.30^(b,c) 24 24 2 2 60 223 24 523 −3.5 1315 0.270 1.804 3.98 879 45.02.31^(b,c) 24 24 2 2 60 223 29 523 −3.5 1315 0.259 1.822 3.88 836 43.62.32^(b,c) 24 24 2 2 60 223 34 523 −3.5 1315 0.251 1.817 3.83 809 47.22.33^(b,c) 24 24 2 2 60 223 33 523 −3.5 1315 0.251 1.817 3.83 822 46.32.34 24 24 4.1 0 60 223 24 523 −3.5 1315 0.279 1.803 4.05 888 43.1 2.3524 24 4.1 0 60 223 28 523 −3.5 1315 0.253 1.812 3.92 887 43.0 2.36 25 280 0 60 223 24 523 −3.5 1315 0.273 1.813 4.00 903 44.7 2.37 25 30 0 0 63223 24 523 −3.5 1315 0.275 1.798 4.03 833 45.0 2.38 25 30 0 0 63 223 28523 −3.5 1315 0.268 1.801 3.97 860 45.8 2.39^(c) 25 30 0 0 63 223 24 523−3.5 1315 0.259 1.797 3.91 942 46.9 2.40 24 24 4.1 3.8 66 223 28 523−3.5 1315 0.261 1.796 3.92 861 44.6 2.41 24 24 4.1 3.8 66 223 24 523−3.5 1315 0.271 1.802 4.00 843 43.1 2.42 24 24 8 0 66 223 24 523 −3.51315 0.269 1.800 3.98 901 43.2 2.43 24 24 8 0 66 223 28 523 −3.5 13150.261 1.799 3.96 735 40.3 2.44 25 30 4.5 3.1 75 223 28 523 −3.5 13150.248 1.802 3.82 877 47.5 2.45 25 30 4.5 3.1 75 223 28 523 −3.5 13150.246 1.798 3.81 872 46.3 2.46 25 30 7.6 3 80 223 24 523 −3.5 1315 0.2401.800 3.76 876 45.1 2.47 25 30 7.6 3 80 223 24 523 −3.5 1315 0.240 1.8003.76 874 45.4 2.48 32 32 2.2 1.1 80 223 24 523 −3.5 1315 0.190 1.8093.33 884 48.7 ^(a)Downstream oven temperature mean lowered to reduceoxidation density of stabilized sample by 1%. ^(b)Downstream oventemperature mean lowered to reduce oxidation density of stabilizedsample by 2%. ^(c)Reduced tension variability. ^(d)Downstream oventemperature mean increased to reduce stabilization time by a factor of2. ^(e)Properties of fiber exiting the low temperature carbonizationfurnace (high temperature furnace turned off).

TABLE 8 Processing Conditions and Property Data for Carbon Fibersprepared from PAN precursor fibers having a denier of 1.33 dpf % Stretchin Example OX-Pass # OX-Str. Temp. TR-Str. Temp. C2-Str. HTT WPULDensity Diameter Strength Modulus No. 1 2 3 4 (%) OX (° C.) (%) TR (°C.) (%) (° C.) (g/m) (g/cm³) (μm) (ksi) (Msi) Comparative 8.3 0 0 0 8.3235 2.7 490 −4.1 1250 0.858 1.790 7.1 620 33.1 3.1 Comparative 8.3 0 0 08.3 223 2.7 523 −3.7 1250 0.849 1.801 7.1 724 36.2 3.2 Comparative 8.3 00 0 8.3 223 2.7 523 −3.5 1250 0.848 1.799 7.1 702 37.1 3.3 Comparative13.7 0 0 0 13.7 223 5 523 −3.5 1250 0.799 1.802 6.9 721 38.3 3.4Comparative 13.7 0 0 0 13.7 223 5 523 −3.5 1315 0.793 1.780 6.9 756 36.03.5 3.6 50 20 0 0 80 223 5 523 −3.5 1315 0.499 1.779 5.5 791 42.1 3.7 5020 5.5 5.3 100 223 18 523 −3.5 1250 0.400 1.806 4.9 766 41.6 3.8 50 205.5 5.3 100 223 18 523 −3.5 1315 0.394 1.788 4.8 773 42.2 3.9 50 20 5.55.3 100 223 24 523 −3.5 1315 0.374 1.792 4.7 781 43.1 3.10 50 20 5.5 5.3100 223 28 523 −3.5 1315 0.328 1.790 4.4 774 47.2

TABLE 9 Processing Conditions and Property Data for Carbon Fibersprepared from PAN precursor fibers having a denier of 1.53 dpf % Stretchin Example OX-Pass # OX-Str. Temp. TR-Str. Temp. C2-Str. HTT WPULDensity Diameter Strength Modulus No. 1 2 3 4 (%) OX (° C.) (%) TR (°C.) (%) (° C.) (g/m) (g/cm³) (μm) (ksi) (Msi) Comparative 8.3 0 0 0 8.3223 2.7 523 −3.7 1250 1.781 1.798 7.2 544 31.2 4.1 Comparative 8.3 0 0 08.3 235 2.7 490 −4.1 1280 3.283 1.785 7.1 516 30.1 4.2 Comparative 8.3 00 0 8.3 223 12 523 −3.5 1315 1.618 1.810 6.9 558 34.0 4.3 4.4 50 16 4.62.2 86 223 12 523 −3.5 1315 0.961 1.785 5.3 519 36.5 4.5 60 13 3.6 2 91223 12 523 −3.5 1315 0.937 1.786 5.3 524 36.8 4.6 60 13 3.6 2 91 223 24523 −2.9 1315 0.826 1.786 5.0 528 40.9 4.7^(a) 60 13 3.6 2 91 223 24 523−3.5 1315 0.882 1.780 5.1 607 45.3 ^(a)Performed at twice the standardcarbonization residence time.

In the Examples in Tables 6 through 9 it can be readily seen thatcontrolled stretching of the carbon fibers in accordance with theinvention results in carbon fibers having improved tensile strength andmodulus of elasticity in comparison to carbon fibers that are preparedaccording to conventional methods. For instance, in Table 6, Example 35shows significant improvement in tensile strength and modulus ofelasticity over a carbon fiber prepared using conventional techniques,such as Comparative Example 1. Specifically, gains in tensile strengthand in modulus of up to ˜20% were achieved by subjecting the 0.6 dpfprecursor to controlled oxidation stretching and subsequentcarbonization at the conditions specified in Example 35. Theseconditions corresponded to between one and two transitions in each ofthe four passes employed. Other conditions led to different combinationsof transitions and, accordingly, different mechanical properties in theresulting carbon fibers, as discussed below.

Comparative Examples 1-4 show that gains in mechanical properties areachievable by improving the mechanical performance of the line,specifically by taking steps to reduce tension variability. For improvedresults, tension variability should preferably be within <5%, and mostpreferably be within <3%. In contrast, Comparative Examples 4-9 showthat enhanced mechanical properties of the order achieved by theinvention cannot be realized by a conventional art approach of applyinga high overall oxidation stretch at a high strain rate early on in theoxidation process.

Examples 1-5 and Comparative Example 6 were run as a set to illustratethe effect of oxidation stretch distribution on the mechanicalproperties of carbon fibers of similar overall oxidation stretch andfilament diameter. For an overall oxidation stretch of 15%, and afilament diameter of ˜4.35 μm, the samples tested show significantincreases in tensile strength and modulus (˜10% and ˜7%, respectively).Since these increases were achieved by varying only the distribution ofoxidation stretches per pass, it follows that oxidation stretching has adirect impact on the mechanical properties of carbon fibers,independently of their filament diameter.

Example 5 also illustrates the effect of applying shrinkage as anintermediate step (in pass #3-4) during oxidation. Comparison withExamples 1-4 shows that allowing the fiber to shrink during anintermediate oxidation step is not detrimental to fiber properties, andcan be used to increase filament diameter. This result illustrates theadvantages of applying tailored combinations of stretch and shrinkageprotocols to produce carbon fibers of desired mechanical properties,diameter and weight per unit length.

Examples 6-61 exemplify a systematic investigation of transitions andtheir impact on the mechanical properties of fibers made from the 0.6dpf precursor at a variety of experimental conditions. Those skilled inthe art will recognize the beneficial effects of parameters such as lowand high temperature carbonization furnace stretches and temperatures onthe results. However, the magnitudes of the stretches involved arerelatively large compared to prior art. Those skilled in the art willalso recognize additional examples of improvements in mechanicalproperties of fibers of similar diameter, achieved only through changesin oxidation stretching distribution parameters (for example, compareExamples 41 and 55).

Examples 42-53 illustrate the effect of increasing the carbonizationtemperature above 1,350° C. in order to increase modulus. As expected,tensile strength decreases with increasing carbonization temperature.However, the tensile strength values remain above 850 ksi, which isunexpected for carbon fibers with modulus up to 58 Msi (see Example 53).

Tables 7-9 confirm that the method of the invention also leads toincreases in mechanical properties when applied to PAN precursors oflarger filament denier (Tables 7-9) and composition (Table 9). Thoseskilled in the art will recognize that some of the combinations ofmechanical properties listed in these Tables are unexpectedly high formaterials made from the larger denier (more economical) precursors.

Many modifications and other embodiments of the invention set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. A method of making a carbon fiber having improved tensile strengthand modulus of elasticity, the method comprising: advancing a carbonfiber precursor polymer through at least 4 passes in an oxidation ovento produce an oxidized fiber, the oven having an oxidizing atmosphere ata temperature between about 175 to 300° C.; subjecting the fiber to a %stretch in a first pass that is between 5 and 30%, a % stretch in asecond pass that is between 5 and 20%, and a % stretch in a third andfourth pass that is between 2 and 15%; passing the oxidized fiberthrough a furnace at a temperature between about 400 and 800° C.; andcarbonizing the fiber by passing the fiber through a furnace having atemperature that is between about 1300 and 1500° C.
 2. The method ofclaim 1, wherein the step of stretching comprises subjecting the fiberin the plurality of passes to a plurality of controlled stretches thatimpose a % stretch on the fiber that is about 25% or less in each of thetwo or more passes.
 3. The method of claim 1, wherein the fiber exitsthe oxidation oven as part of a pass.
 4. The method of claim 1, whereinthe fiber comprise one or more comonomers selected from the groupconsisting of acrylonitrile, methyl acrylate, methacrylic acid, sodiummethallylsulfonate, and itaconic acid.
 5. The method of claim 1, whereinthe precursor fiber has a denier between about 0.6 to 0.8 dpf.
 6. Themethod of claim 1, further comprising the steps of surface treating andsizing the precursor fiber.
 7. A method of stretching a fiber comprisinga carbon fiber precursor polymer, the method comprising: advancing afiber through an oxidation oven in two or more passes, the oven havingan oxidizing atmosphere; stretching the fiber in two or more of thepasses in an amount sufficient to cause the fiber to undergo at leasttwo transitions in each of the two or more passes, wherein eachtransition comprises a region of inflection that is determinable from aplot of tension versus % stretch.
 8. The method of claim 7, wherein thestep of stretching comprises subjecting the fiber in the two or morepasses to a plurality of controlled stretches that impose a % stretch onthe fiber that is about 25% or less in each of the two or more passes.9. The method of claim 8, further comprising the step of permitting thefiber to stabilize between each controlled stretch before subjecting thefiber to a further stretch.
 10. The method of claim 9, wherein the stepof stretching comprises subjecting the fiber in each of the two or morepasses to a plurality of controlled stretches that impose a % stretch onthe fiber that between about 10 and 25% in the two or more passes. 11.The method of claim 7, further comprising the steps of: stretching thefiber in a first pass in an amount sufficient to cause the fiber toundergo at least two transitions, wherein each transition comprises aninflection point that is obtainable from a plot of tension versus %stretch; and stretching the fiber in a second pass in an amountsufficient to cause the fiber to undergo at least two transitions,wherein each transition comprises an inflection point that is obtainablefrom a plot of tension versus % stretch, and wherein the fiber undergoesa cumulative stretch in both the first and second passes that is atleast about 30% or greater.
 12. The method of claim 11, wherein thefiber in stretched in the first pass in an amount sufficient to causethe fiber to undergo at least three transitions.
 13. The method of claim7, wherein the fiber exits the oxidation oven as part of a pass.
 14. Themethod of claim 7, wherein the fiber makes three or more passes throughthe oxidation oven, and wherein in each of said passes the fiber issubjected to stretching in an amount sufficient to cause the fiber toundergo at least two transitions.
 15. The method of claim 7, wherein thefiber is exposed to a strain rate that is no greater than about 10% perminute per pass.
 16. The method of claim 7, wherein the fiber isintroduced into a plurality of oxidation ovens and wherein eachsuccessive furnace includes an oxidizing atmosphere that is at atemperature that is as least as great as a preceding oxidation oven. 17.The method of claim 7, wherein the fiber is exposed to a cumulativestretch that is sufficient to cause the fiber to have a 25 to 50%reduction in the fiber's diameter.
 18. The method of claim 7, whereinstretching the fiber further comprises subjecting the fiber to aplurality of controlled stretches in the oxidation oven wherein: a) in afirst pass the fiber is subjected to a % stretch that is between 10 and40%; b) in a second pass the fiber is subjected to a % stretch that isbetween about 2 and 20%; c) in a third pass the % stretch is betweenabout 2 and 16%; and d) in a fourth pass the fiber is subjected to a %stretch that is between about 2 and 12%.
 19. A method of producing acarbon fiber comprising: introducing a fiber comprising apolyacrylonitrile polymer into an oxidation oven having an oxidizingatmosphere at a temperature between about 175 to 300° C., the oxidationoven having a plurality of cooperating pairs of rolls that each define afiber pass through the furnace; stretching the fiber as it passesthrough two or more of said fiber passes in an amount sufficient tocause the fiber to undergo one or more transitions per pass, whereineach transition comprises an inflection point that is obtainable from aplot of tension versus % stretch; advancing the oxidized fiber out ofthe oven; and subjecting the oxidized fiber to a carbonization heattreatment.
 20. The method according to claim 19, wherein stretching thefiber further comprises subjecting the fiber to a plurality ofcontrolled stretches in the oxidation oven wherein: a) in a first passthe fiber is subjected to a % stretch that is between 10 and 40%; b) ina second pass the fiber is subjected to a % stretch that is betweenabout 2 and 20%; c) in a third pass the % stretch is between about 2 and16%; and d) in a fourth pass the fiber is subjected to a % stretch thatis between about 2 and 12%.
 21. The method of claim 19, wherein thefiber comprise one or more comonomers selected from the group consistingof acrylonitrile, methyl acrylate, methacrylic acid, sodiummethallylsulfonate, and itaconic acid.
 22. The method of claim 19,wherein the fiber has a denier between about 0.6 to 1.53 dpf.
 23. Themethod according to claim 19, wherein the fiber makes at least 8 passesthrough the oxidation oven and wherein in each successive pass the %stretch is less than that in a preceding pass.
 24. The method accordingto claim 19, wherein the carbonization heat treatment comprisesadvancing the fiber through a high temperature furnace that is at atemperature between about 1200 and 2000° C.
 25. The method according toclaim 24, wherein the high temperature furnace is at a temperaturebetween about 1300 and 1500° C.
 26. The method according to claim 24,further comprising advancing the fiber through a low temperature furnaceprior to passing the fiber through the high temperature furnace, andwherein the low temperature furnace is at a temperature between about400 and 800° C.
 27. The method according to claim 26, further comprisingstretching the fiber as it advances through the low temperature furnace,and wherein the amount stretching is between 1 and 24% stretch.
 28. Themethod according to claim 19, wherein upon exiting the oxidation oven,the fiber has an average diameter that is between 10 and 50% less thanthe fiber's original diameter prior to entering the oxidation oven. 29.The method according to claim 19, wherein the fiber comprises a fiberbundle having between about 1,000 and 50,000 individual filaments. 30.The method according to claim 19, wherein the fiber makes three or morepasses through the oxidation oven, and wherein in each of said passesthe fiber is subjected to stretching in an amount sufficient to causethe fiber to undergo at least two transitions.
 31. A carbon fiber havinga tensile strength from about 950 to 1166 ksi and modulus of elasticityof about 45 to 53.3 Msi, and wherein the carbon fiber has a crystallitewidth value L_(a) of about 4 nm or greater.
 32. The carbon fiberaccording to claim 31, wherein the carbon fiber has an atomic forcemicroscopy surface image that is characterized by the presence of aplurality of striations of low phase angle domains and a plurality ofstriations of high phase angle domains that extend across the surface ofthe carbon fiber.
 33. The carbon fiber according to claim 31, whereinthe carbon fiber has an average phase angle depth of 5 nanometers orgreater.
 34. The carbon fiber according to claim 31, wherein the carbonfiber has a tensile strength about 1000 to 1166 ksi and modulus ofelasticity of about 50 to 53.3 Msi.
 35. The carbon fiber according toclaim 31, wherein the carbon fiber has a roughness having a Root MeanSquare Roughness (Rq) value of about 2 or greater.
 36. A carbon fiberhaving a tensile strength from about 950 to 1166 ksi and modulus ofelasticity of about 45 to 53.3 Msi.
 37. The carbon fiber of claim 36,wherein the carbon fiber has a resistivity value of about 14 μΩm orgreater
 38. The carbon fiber of claim 36, wherein the carbon fiber has aradial uniformity RU value of about 50 or greater.
 39. The carbon fiberof claim 36, wherein the carbon fiber has an average phase angle depthof 8 nanometers or greater.
 40. The carbon fiber according to claim 36,wherein the carbon fiber has a tensile strength about 1000 to 1166 ksiand modulus of elasticity of about 50 to 53.3 Msi.
 41. A carbon fiberhaving a Root Mean Square Roughness (Rq) value of about 2 or greater, anArithmetic Average Roughness (Ra) of about 2.5 or greater, and aresistivity of at least 13 μΩm or greater.
 42. The carbon fiber of claim41, wherein the carbon fiber has a radial uniformity RU value of about50 or greater.
 43. The carbon fiber of claim 41, wherein the carbonfiber has an average phase angle depth of 5 nanometers or greater. 44.The carbon fiber of claim 41, wherein the carbon fiber has a crystallitewidth value L_(a) of about 4 nm or greater.
 45. The carbon fiber ofclaim 41, wherein the carbon fiber has an RD value of about 0.01 to0.05.